deps: update dependency com.google.protobuf:protobuf-bom to v3.21.8 (#872)

Author: renovate-bot

first-party-dependencies/pom.xml

@@ -60,7 +60,7 @@
     <grpc-gcp.version>1.3.0</grpc-gcp.version>
     <gson.version>2.9.1</gson.version>
     <guava.version>31.1-jre</guava.version>
-    <protobuf.version>3.21.7</protobuf.version>
+    <protobuf.version>3.21.8</protobuf.version>
     <google.api-common.version>2.2.1</google.api-common.version>
     <google.common-protos.version>2.9.6</google.common-protos.version>
     <google.core.version>2.8.21</google.core.version>