[openrewrite] add UpgradeToJava21
          #2644
        
          
      
                
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follow up:
tech.picnic.errorprone.refasterrules#2641UpgradeToJava17#2636rewritesupport forerrorprone.refasterrules#2634rewritesupport forerrorprone.refasterrulescheckstyle/checkstyle#17487rewritesupport forerrorprone.refasterrulesspring-projects/spring-boot#46749Rewritecovering S1144: Unused "private" methods should be removed checkstyle/checkstyle#17545rewritesupport forerrorprone.refasterrulespmd/pmd#5956Rewrite & PMDcovering S1144: Unused "private" methods should be removed opensearch-project/OpenSearch#18791rewritesupport forerrorprone.refasterrules#2576rewritesupport forerrorprone.refasterrulesapache/kafka#20219