deps: update dependency com.google.protobuf:protobuf-bom to v3.15.0 (#116)

Author: renovate-bot

pom.xml

@@ -64,7 +64,7 @@
     <github.global.server>github</github.global.server>
     <site.installationModule>google-iam-parent</site.installationModule>
     <google.common-protos.version>1.17.0</google.common-protos.version>
-    <protobuf.version>3.14.0</protobuf.version>
+    <protobuf.version>3.15.0</protobuf.version>
     <junit.version>4.13.2</junit.version>
     <grpc.version>1.35.0</grpc.version>
     <guava.version>30.1-android</guava.version>