Upgrade Checkstyle 10.20.2 -> 10.21.0

[Picnic-DevPla-Bot]

This PR contains the following updates:

Package	Type	Update	Change
Checkstyle (source)	build	minor	10.20.2 -> 10.21.0

---

• If you want to rebase/retry this PR, check this box

[Picnic-DevPla-Bot]

Suggested commit message:

Upgrade Checkstyle 10.20.2 -> 10.21.0 (#1460)

While there, also update the integration test.

See:
- https://checkstyle.sourceforge.io/releasenotes.html
- https://github.com/checkstyle/checkstyle/releases/tag/checkstyle-10.21.0
- https://github.com/checkstyle/checkstyle/compare/checkstyle-10.20.2...checkstyle-10.21.0

[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.

[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.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud