deps: update core dependencies (#86)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [io.grpc:grpc-bom](https://togithub.com/grpc/grpc-java) | patch | `1.28.0` -> `1.28.1` |
| [com.google.api:gax-bom](https://togithub.com/googleapis/gax-java) | minor | `1.55.0` -> `1.56.0` |

---

### Release Notes

<details>
<summary>grpc/grpc-java</summary>

### [`v1.28.1`](https://togithub.com/grpc/grpc-java/releases/v1.28.1)

[Compare Source](https://togithub.com/grpc/grpc-java/compare/v1.28.0...v1.28.1)

#### Bug Fixes

-   xds: implement more clear specifications for locality filtering (allow localities with 0 endpoint, ignore localities with 0 weight, check for locality priority continuity). ([#&#8203;6875](https://togithub.com/grpc/grpc-java/issues/6875))

</details>

<details>
<summary>googleapis/gax-java</summary>

### [`v1.56.0`](https://togithub.com/googleapis/gax-java/releases/v1.56.0)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v1.55.0...v1.56.0)

04-06-2020 10:29 PDT

##### New Features

-   Add ability to partially update retry settings ([#&#8203;993](https://togithub.com/googleapis/gax-java/pull/993))

##### Dependencies

-   Update grpc to 1.28.1 ([#&#8203;1013](https://togithub.com/googleapis/gax-java/pull/1013))
-   Update com.google.api:api-common to 1.9.0 ([#&#8203;992](https://togithub.com/googleapis/gax-java/pull/992))
-   Add javax.annotations dependency in bazel build to gax-grpc ([#&#8203;1000](https://togithub.com/googleapis/gax-java/pull/1000))

##### Internal / Testing Changes

-   test: make it easier to debug failures in SettingsTest ([#&#8203;994](https://togithub.com/googleapis/gax-java/pull/994))
-   build: update dependency gradle.plugin.com.dorongold.plugins:task-tree to v1.5 ([#&#8203;995](https://togithub.com/googleapis/gax-java/pull/995))
-   test: update dependency com.google.api.grpc:grpc-google-cloud-pubsub-v1 to v1.86.0 ([#&#8203;1008](https://togithub.com/googleapis/gax-java/pull/1008))

</details>

---

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pom.xml

@@ -155,8 +155,8 @@
     <google.core.version>1.93.3</google.core.version>
     <google.api-common.version>1.9.0</google.api-common.version>
     <google.common-protos.version>1.17.0</google.common-protos.version>
-    <gax.version>1.55.0</gax.version>
-    <grpc.version>1.28.0</grpc.version>
+    <gax.version>1.56.0</gax.version>
+    <grpc.version>1.28.1</grpc.version>
     <protobuf.version>3.11.4</protobuf.version>
     <junit.version>4.13</junit.version>
     <guava.version>28.2-android</guava.version>