chore: add grpc-experimental branch rules (googleapis#1745)

Enable github branch rules for gapic client branch [grpc-experimental](https://togithub.com/googleapis/java-datastore/tree/grpc-experimental).
We need the dependencies to be up-to-date before launching the public preview.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [x] Make sure to open an issue as a [bug/issue](https://togithub.com/googleapis/java-datastore/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> ☕️

Author: cindy-peng

.github/dependabot.yml

@@ -17,4 +17,15 @@ updates:
     # If a security vulnerability comes in, we will be notified about 
     # it via template in the synthtool repository.
     ignore:
-      - dependency-name: "*"
+    - dependency-name: "*"
+
+# rules for the `grpc-experimental` branch
+  - package-ecosystem: maven
+    directory: "/"
+    schedule:
+      interval: daily
+    commit-message:
+      # Prefix all commit messages with "deps: "
+      prefix: "deps"
+    open-pull-requests-limit: 10
+    target-branch: "grpc-experimental"

.github/sync-repo-settings.yaml

@@ -109,6 +109,22 @@ branchProtectionRules:
       - cla/google
       - 'Kokoro - Test: Java GraalVM Native Image'
       - 'Kokoro - Test: Java 17 GraalVM Native Image'
+  - pattern: grpc-experimental
+    isAdminEnforced: true
+    requiredApprovingReviewCount: 1
+    requiresCodeOwnerReviews: true
+    requiresStrictStatusChecks: false
+    requiredStatusCheckContexts:
+        - dependencies (17)
+        - lint
+        - clirr
+        - units (8)
+        - units (11)
+        - 'Kokoro - Test: Integration'
+        - cla/google
+        - 'Kokoro - Test: Java GraalVM Native Image'
+        - 'Kokoro - Test: Java 17 GraalVM Native Image'
+        - javadoc
   - pattern: 2.15.x
     isAdminEnforced: true
     requiredApprovingReviewCount: 1