deps: update dependency com.google.protobuf:protobuf-bom to v3.21.6 (#797)

Author: renovate-bot

first-party-dependencies/pom.xml

@@ -59,7 +59,7 @@
     <gax.version>2.19.1</gax.version>
     <grpc-gcp.version>1.2.1</grpc-gcp.version>
     <guava.version>31.1-jre</guava.version>
-    <protobuf.version>3.21.5</protobuf.version>
+    <protobuf.version>3.21.6</protobuf.version>
     <google.api-common.version>2.2.1</google.api-common.version>
     <google.common-protos.version>2.9.2</google.common-protos.version>
     <google.core.version>2.8.11</google.core.version>