Bump checkstyle

Bumps [checkstyle](https://github.com/checkstyle/checkstyle) from 6.16 to 8.18.
- [Release notes](https://github.com/checkstyle/checkstyle/releases)
- [Commits](https://github.com/checkstyle/checkstyle/compare/checkstyle-6.16...checkstyle-8.18)

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