chore: change branch master to main in github configurations (#550)

Author: Neenu1995

.github/sync-repo-settings.yaml

@@ -2,7 +2,7 @@ rebaseMergeAllowed: false
 squashMergeAllowed: true
 mergeCommitAllowed: false
 branchProtectionRules:
-  - pattern: master
+  - pattern: main
     isAdminEnforced: true
     requiredApprovingReviewCount: 1
     requiresCodeOwnerReviews: true

README.md

@@ -213,27 +213,27 @@ Testing
 
 This library has tools to help write tests for code that uses the Datastore.
 
-See [TESTING.md](https://github.com/googleapis/google-cloud-java/blob/master/TESTING.md#testing-code-that-uses-datastore) to read more about testing.
+See [TESTING.md](https://github.com/googleapis/google-cloud-java/blob/main/TESTING.md#testing-code-that-uses-datastore) to read more about testing.
 
 Example Applications
 --------------------
-- [`Bookshelf`](https://github.com/GoogleCloudPlatform/getting-started-java/tree/master/bookshelf) - An App Engine app that manages a virtual bookshelf.
+- [`Bookshelf`](https://github.com/GoogleCloudPlatform/getting-started-java/tree/main/bookshelf) - An App Engine app that manages a virtual bookshelf.
   - This app uses `google-cloud` to interface with Cloud Datastore and Cloud Storage. It also uses Cloud SQL, another Google Cloud Platform service.
-- [`Flexible Environment/Datastore example`](https://github.com/GoogleCloudPlatform/java-docs-samples/tree/master/flexible/datastore) - A simple app that uses Cloud Datastore to list the last 10 IP addresses that visited your site.
-- [`SparkDemo`](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/master/flexible/sparkjava) - An example of using `google-cloud-datastore` from within the SparkJava and App Engine Flexible Environment frameworks.
-  - Read about how it works on the example's [README page](https://github.com/GoogleCloudPlatform/java-docs-samples/tree/master/flexible/sparkjava#how-does-it-work).
+- [`Flexible Environment/Datastore example`](https://github.com/GoogleCloudPlatform/java-docs-samples/tree/main/flexible/datastore) - A simple app that uses Cloud Datastore to list the last 10 IP addresses that visited your site.
+- [`SparkDemo`](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/main/flexible/sparkjava) - An example of using `google-cloud-datastore` from within the SparkJava and App Engine Flexible Environment frameworks.
+  - Read about how it works on the example's [README page](https://github.com/GoogleCloudPlatform/java-docs-samples/tree/main/flexible/sparkjava#how-does-it-work).
 
 
 
 
 ## Samples
 
-Samples are in the [`samples/`](https://github.com/googleapis/java-datastore/tree/master/samples) directory.
+Samples are in the [`samples/`](https://github.com/googleapis/java-datastore/tree/main/samples) directory.
 
 | Sample                      | Source Code                       | Try it |
 | --------------------------- | --------------------------------- | ------ |
-| Quickstart Sample | [source code](https://github.com/googleapis/java-datastore/blob/master/samples/snippets/src/main/java/com/example/datastore/QuickstartSample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-datastore&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/datastore/QuickstartSample.java) |
-| Task List | [source code](https://github.com/googleapis/java-datastore/blob/master/samples/snippets/src/main/java/com/google/datastore/snippets/TaskList.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-datastore&page=editor&open_in_editor=samples/snippets/src/main/java/com/google/datastore/snippets/TaskList.java) |
+| Quickstart Sample | [source code](https://github.com/googleapis/java-datastore/blob/main/samples/snippets/src/main/java/com/example/datastore/QuickstartSample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-datastore&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/datastore/QuickstartSample.java) |
+| Task List | [source code](https://github.com/googleapis/java-datastore/blob/main/samples/snippets/src/main/java/com/google/datastore/snippets/TaskList.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-datastore&page=editor&open_in_editor=samples/snippets/src/main/java/com/google/datastore/snippets/TaskList.java) |
 
 
 
@@ -342,10 +342,10 @@ Java is a registered trademark of Oracle and/or its affiliates.
 [developer-console]: https://console.developers.google.com/
 [create-project]: https://cloud.google.com/resource-manager/docs/creating-managing-projects
 [cloud-sdk]: https://cloud.google.com/sdk/
-[troubleshooting]: https://github.com/googleapis/google-cloud-common/blob/master/troubleshooting/readme.md#troubleshooting
-[contributing]: https://github.com/googleapis/java-datastore/blob/master/CONTRIBUTING.md
-[code-of-conduct]: https://github.com/googleapis/java-datastore/blob/master/CODE_OF_CONDUCT.md#contributor-code-of-conduct
-[license]: https://github.com/googleapis/java-datastore/blob/master/LICENSE
+[troubleshooting]: https://github.com/googleapis/google-cloud-common/blob/main/troubleshooting/readme.md#troubleshooting
+[contributing]: https://github.com/googleapis/java-datastore/blob/main/CONTRIBUTING.md
+[code-of-conduct]: https://github.com/googleapis/java-datastore/blob/main/CODE_OF_CONDUCT.md#contributor-code-of-conduct
+[license]: https://github.com/googleapis/java-datastore/blob/main/LICENSE
 
 [enable-api]: https://console.cloud.google.com/flows/enableapi?apiid=datastore.googleapis.com
 [libraries-bom]: https://github.com/GoogleCloudPlatform/cloud-opensource-java/wiki/The-Google-Cloud-Platform-Libraries-BOM