upgrade protobuf due to CVEs - to v3.21.7 (bump from v3.20.1)

[pjfanning]

• https://mvnrepository.com/artifact/com.google.protobuf/protobuf-java/3.20.1 (3 CVEs)
• https://mvnrepository.com/artifact/com.google.protobuf/protobuf-java/3.20.3 (no CVEs)
• https://mvnrepository.com/artifact/com.google.protobuf/protobuf-java/3.21.7 (no CVEs)
• our sbt build actually uses 3.21.1 because it is a transitive dependency of io.grpc:grpc-protobuf:1.48.1
• so we will need to bump our version higher again. 3.21.7 is the lowest bump that gets us to a jar with no CVEs
• Pekko core has a shaded protobuf-v3 jar based on protobuf 3.16 - but there is no reason to suddenly align these since they have been different for a while

One sbt-scripted test fails - gen-scala-server/06-compatibility-plugins

• this test appears to rely on ProtocJSPlugin and protobuf js support is now separate from java support (protobuf 3.21+)