deps: update dependency com.google.protobuf:protobuf-java to v3.12.0 (#130)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| com.google.protobuf:protobuf-java | minor | `3.11.4` -> `3.12.0` |

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pom.xml

@@ -68,7 +68,7 @@
     <google.common-protos.version>1.18.0</google.common-protos.version>
     <gax.version>1.56.0</gax.version>
     <grpc.version>1.29.0</grpc.version>
-    <protobuf.version>3.11.4</protobuf.version>
+    <protobuf.version>3.12.0</protobuf.version>
     <junit.version>4.13</junit.version>
     <guava.version>29.0-android</guava.version>
     <threeten.version>1.4.4</threeten.version>