Upgrade maven-site-plugin 3.20.0 -> 3.21.0

[Picnic-DevPla-Bot]

This PR contains the following updates:

Package	Type	Update	Change
maven-site-plugin	build	minor	3.20.0 -> 3.21.0

---

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

[Picnic-DevPla-Bot]

Suggested commit message:

Upgrade maven-site-plugin 3.20.0 -> 3.21.0 (#1371)

See:
- https://issues.apache.org/jira/issues/?jql=project%20%3D%20MSITE%20AND%20fixVersion%20%3E%203.20.0%20AND%20fixVersion%20%3C%3D%203.21.0
- https://github.com/apache/maven-site-plugin/compare/maven-site-plugin-3.20.0...maven-site-plugin-3.21.0

[github-actions[bot]]

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[bot]]

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[bot]]

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[bot]]

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[bot]]

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[bot]]

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 SonarCloud

[github-actions[bot]]

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.