[openrewrite] add UpgradeToJava21

[Pankraz76]

follow up:

• [openrewrite] add tech.picnic.errorprone.refasterrules #2641
• [openrewrite] initial merge and UpgradeToJava17 #2636
• Add rewrite support for errorprone.refasterrules #2634
• Add rewrite support for errorprone.refasterrules checkstyle/checkstyle#17487
• Add rewrite support for errorprone.refasterrules spring-projects/spring-boot#46749
• Issue #17487: Introduce Rewrite covering S1144: Unused "private" methods should be removed checkstyle/checkstyle#17545
• Fix #5953: [java] PoC: Add rewrite support for errorprone.refasterrules pmd/pmd#5956
• Introduce Rewrite & PMD covering S1144: Unused "private" methods should be removed opensearch-project/OpenSearch#18791
• PoC: Add rewrite support for errorprone.refasterrules #2576
• PoC: Add rewrite support for errorprone.refasterrules apache/kafka#20219

[Pankraz76]

autobuild:

[2025-09-25 09:05:40] [autobuild] Caused by: java.lang.IllegalArgumentException: error: release version 21 not supported

[nedtwigg]

• slow down, don't open a new rewrite PR until the last one is merged
• you are linking to many many things which don't seem relevant to this project.
• try to be communicate in a way which is focused on the project you are asking to merge changes into

[Pankraz76]

• rewrite PR

of course im sorry. The benefit is; now we can be parallel, as its different topic and reproducable.