Fix usage of javax.annotation

[joschi]

For many JVM-based projects, the @javax.annotation.Generated annotation is being used.

Starting with Java 9 and higher, this class is not shipped with the JDK anymore and requires an explicit dependency on javax.annotation:javax.annotation-api which was missing in most of the generators.

This PR adds the dependency in the respective POM, build.gradle, and build.sbt templates and cleans up the usage of the hideGenerationTimestamp property across templates.

PR checklist

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[wing328]

Thanks for the PR, which looks good to me 👍