Currently, Checkstyle is using JDK 11 as the baseline version. However, Java has evolved significantly, and most of the ecosystem has now moved towards JDK 21.

This issue proposes to incrementally upgrade Checkstyle’s baseline JDK version in two stages:

• move all Inputs for JDK 12-17 to compiled folder ✅
• update all CIs to use jdk17, there should be no jdk11 be left. Still not updates in pom.xml for required jdk. ✅
• send PR to upgrade pom.xml to min required jdk, to see CI and make further plan. ✅
• update pom.xml - it should be separate issue as it is big breaking change and it would be visible in release notes clearly. Version will be 11.0.0 ✅
• revert changes in pom.xml to make Inputs be compiled by maven.
• review all PRs with dependencies and prepare them for merge, may be just rebase in UI will be required.