[chore] Release 6.16.0 (firebase#471)

[chore] Release 6.15.0 (firebase#457)

[chore] Release 6.14.0 (firebase#444)

[chore] Release 6.13.0 take 4 (firebase#416)

[maven-release-plugin] copy for tag v6.12.2

[maven-release-plugin] copy for tag v6.12.1

[maven-release-plugin] copy for tag v6.12.0

[maven-release-plugin] copy for tag v6.11.0

[maven-release-plugin] copy for tag v6.10.0

[maven-release-plugin] copy for tag v6.9.0