deps: update core dependencies (#2)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.api:gax-grpc](https://github.com/googleapis/gax-java) | minor | `1.55.0` -> `1.56.0` |
| [com.google.api:gax-bom](https://github.com/googleapis/gax-java) | minor | `1.55.0` -> `1.56.0` |
| [io.grpc:grpc-bom](https://github.com/grpc/grpc-java) | minor | `1.27.2` -> `1.28.1` |

---

### Release Notes

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

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

[Compare Source](https://github.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://github.com/googleapis/gax-java/pull/993))

##### Dependencies

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

##### Internal / Testing Changes

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

</details>

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

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

[Compare Source](https://github.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://github.com/grpc/grpc-java/issues/6875))

### [`v1.28.0`](https://github.com/grpc/grpc-java/releases/v1.28.0)

[Compare Source](https://github.com/grpc/grpc-java/compare/v1.27.2...v1.28.0)

#### gRPC Java 1.28.0 Release Notes

#### API Changes

-   core: service config parsing now validates the final config object via `LoadBalancerProvider#parseLoadBalancingPolicyConfig`. LoadBalancer developers should use parsed config(`loadBalancingPolicyConfig`) in `ResolvedAddresses` instead of deprecated attribute `io.grpc.LoadBalancer.ATTR_LOAD_BALANCING_CONFIG`.
-   core: remove stickiness from round robin ([#&#8203;6698](https://github.com/grpc/grpc-java/issues/6698))

#### Behavioral Changes

-   core, grpclb: change policy selection strategy for Grpclb policy (take one: eliminate special logic for deciding grpclb policy in core) ([#&#8203;6637](https://github.com/grpc/grpc-java/issues/6637))
    -   gRPCLB policy selection is being stabilized according to [this gRFC](https://github.com/grpc/proposal/blob/master/A26-grpclb-selection.md). gRPCLB policy will not be automatically selected even if there are SRV addresses resolved. You need to explicitly specify the selection by service config (in TXT records, or via `ManagedChannelBuilder#defaultServiceConfig` or `ManagedChannelBuilder#defaultLoadBalancingPolicy`).

#### New Features

-   This release adds an experimental client side implementation of [xDSv2](https://www.envoyproxy.io/docs/envoy/latest/api-v2/api) APIs. This allows a gRPC client written in Java to receive configuration from an xDSv2 API compatible server and use that configuration to load balance RPCs. In this release, only the virtual host matching and cluster route action is supported. More features will be added in future.

#### Bug Fixes

-   netty: fix a deadlock on start gRPC server ([#&#8203;6601](https://github.com/grpc/grpc-java/issues/6601))
-   netty: negotiated application level protocol is now checked against the correct ALPN list in the SslContext settings
-   okhttp&#x3A; fix incorrect connection-level flow control handling at beginning of connection. 
-   alts: Fix using the wrong way of checking if Conscrypt is available ([#&#8203;6672](https://github.com/grpc/grpc-java/issues/6672))
-   grpclb: grpclb can handle empty address from LoadBalancer instead of throwing exception
-   alts: do not exclude opencensus-api from google-oauth2's transitive dependencies ([#&#8203;6607](https://github.com/grpc/grpc-java/issues/6607))
    -   grpc-alts was broken since 1.27.0 release for missing opencensus-api used in google-oauth2. This was due to opencensus-api dependency was eliminated from grpc-core. Now it should be included.

#### Documentation

-   stub,examples: Clarify CallStreamObserver's Javadoc ([#&#8203;6561](https://github.com/grpc/grpc-java/issues/6561))

#### Dependencies

-   Bump Netty to 4.1.45.Final and tcnative to 2.0.28.Final
-   Bump okhttp to 2.7.4
-   Bump google-auth-library-credentials and google-auth-library-oauth2-http to 0.20.0

#### Acknowledgements

[@&#8203;chrisschek](https://github.com/chrisschek) 
[@&#8203;cushon](https://github.com/cushon) Liam Miller-Cushon
[@&#8203;elharo](https://github.com/elharo) Elliotte Rusty Harold
[@&#8203;gfelbing](https://github.com/gfelbing) Georg Welzel
[@&#8203;hatahet](https://github.com/hatahet) Ziad Hatahet
[@&#8203;leventov](https://github.com/leventov) Roman Leventov
[@&#8203;sourabhsparkala](https://github.com/sourabhsparkala) Sourabh Sarvotham Parkala
[@&#8203;ZhenLian](https://github.com/ZhenLian) Zhen Lian

</details>

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pom.xml

@@ -55,8 +55,8 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <site.installationModule>${project.artifactId}</site.installationModule>
 
-    <grpc.version>1.27.2</grpc.version>
-    <gax.version>1.55.0</gax.version>
+    <grpc.version>1.28.1</grpc.version>
+    <gax.version>1.56.0</gax.version>
     <guava.version>28.2-android</guava.version>
     <protobuf.version>3.11.4</protobuf.version>
     <google.api-common.version>1.9.0</google.api-common.version>