chore(deps): update dependencies (#18)

All updates:
| Package | Update | Change |
|---|---|---|
| [com.google.cloud:google-cloud-core-grpc](https://togithub.com/googleapis/google-cloud-java/tree/master/google-cloud-clients/google-cloud-core-grpc) | patch | `1.91.2` -> `1.91.3` |
| [com.google.api:gax-bom](https://togithub.com/googleapis/gax-java) ([source](https://togithub.com/googleapis/gax-java)) | minor | `1.49.0` -> `1.50.1` |
| [com.google.api:gax-grpc](https://togithub.com/googleapis/gax-java) ([source](https://togithub.com/googleapis/gax-java)) | minor | `1.49.0` -> `1.50.1` |
| [io.grpc:grpc-core](https://togithub.com/grpc/grpc-java) ([source](https://togithub.com/grpc/grpc-java)) | minor | `1.24.0` -> `1.25.0` |
| [io.grpc:grpc-bom](https://togithub.com/grpc/grpc-java) ([source](https://togithub.com/grpc/grpc-java)) | minor | `1.24.0` -> `1.25.0` |

**manually copied and merged from the following PRs**
* #1 chore(deps): update dependency com.google.cloud:google-cloud-core-grpc to v1.91.3
* #4 chore(deps): update gax.version to v1.50.1
* #5 chore(deps): update grpc packages to v1.25.0

Author: BenWhitehead

pom.xml

@@ -152,11 +152,11 @@
     <github.global.server>github</github.global.server>
     <site.installationModule>google-cloud-firestore-parent</site.installationModule>
     <project.javadoc.protobufBaseURL>https://googleapis.dev/java/google-api-grpc/latest</project.javadoc.protobufBaseURL>
-    <google.core.version>1.91.2</google.core.version>
+    <google.core.version>1.91.3</google.core.version>
     <google.api-common.version>1.8.1</google.api-common.version>
     <google.common-protos.version>1.17.0</google.common-protos.version>
-    <gax.version>1.49.0</gax.version>
-    <grpc.version>1.24.0</grpc.version>
+    <gax.version>1.50.1</gax.version>
+    <grpc.version>1.25.0</grpc.version>
     <protobuf.version>3.10.0</protobuf.version>
     <junit.version>4.12</junit.version>
     <guava.version>28.1-android</guava.version>