chore(deps): update dependency com.google.protobuf:protobuf-java to v3.15.7 (#1758)

Author: renovate-bot

pom.xml

@@ -486,7 +486,7 @@
     <project.oauth.version>1.31.4</project.oauth.version>
     <project.jsr305.version>3.0.2</project.jsr305.version>
     <project.gson.version>2.8.6</project.gson.version>
-    <project.protobuf-java.version>3.15.6</project.protobuf-java.version>
+    <project.protobuf-java.version>3.15.7</project.protobuf-java.version>
     <project.guava.version>30.1-android</project.guava.version>
     <project.appengine.version>1.9.84</project.appengine.version>
     <project.xpp3.version>1.1.4c</project.xpp3.version>