Upgrade Checkstyle 10.8.1 -> 10.9.1

[Picnic-Bot]

This PR contains the following updates:

Package	Type	Update	Change
com.puppycrawl.tools:checkstyle (source)	build	minor	10.8.1 -> 10.9.1

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[Picnic-Bot]

Suggested commit message:

Upgrade Checkstyle 10.8.1 -> 10.9.1 (#535)

See:
- https://checkstyle.sourceforge.io/releasenotes.html
- https://github.com/checkstyle/checkstyle/releases/tag/checkstyle-10.9.0
- https://github.com/checkstyle/checkstyle/releases/tag/checkstyle-10.9.1
- https://github.com/checkstyle/checkstyle/compare/checkstyle-10.8.1...checkstyle-10.9.1

[github-actions]

Looks good. No mutations were possible for these changes.
Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[github-actions]

Looks good. No mutations were possible for these changes.
Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[github-actions]

Looks good. No mutations were possible for these changes.
Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[Stephan202]

Version 10.9.1 also LGTM.