Bump access-modifier-annotation from 1.16 to 1.21

Bumps [access-modifier-annotation](https://github.com/jenkinsci/pom) from 1.16 to 1.21.
- [Release notes](https://github.com/jenkinsci/pom/releases)
- [Changelog](https://github.com/jenkinsci/pom/blob/master/CHANGELOG.md)
- [Commits](https://github.com/jenkinsci/pom/compare/jenkins-1.16...jenkins-1.21

)

Signed-off-by: dependabot[bot] <support@github.com>