This version works around a warning about DuplicateStrategy in Gradle 6
that will be an error in Gradle 7 caused by [a bug in the plugin][1].
Bumping the version makes a clean build with `--warning-mode all` (at
least if skipping Android and codegen).
[1]: https://github.com/google/protobuf-gradle-plugin/issues/470
javax.annotation-api is licensed CDDL, which was not noticed when it was
introduced. Tomcat provides an Apache 2 version of the same annotation. Note
that this annotation is only used when compiling with Java 9+.
Unfortunately this may cause classpath collisions since there are _many_ copies
of this annotation on Maven Central; we wanted one canonical source and
javax.annotation-api seemed like that source. We hope this won't impact many
users since we have always suggested using it only for compilation. But it will
probably impact some users. However, we didn't create this mess, this seems to
be "standard practice" for J2EE, which this annotation is now part of, so we're
just impacted by it.
Fixes#6833