Remove adopt-openj9

[elharo]

adopt-openj9 is not available and this breaks the CI

[slawekjaranowski]

CI is green, and build by

https://github.com/apache/maven-compiler-plugin/actions/runs/33017149395/job/98340054634#logs

Apache Maven 4.0.0-rc-6 (6a8189b24518daa120539fa41ce12f2b48ec09a8)
Maven home: C:\Users\runneradmin\.m2\wrapper\dists\apache-maven-4.0.0-rc-6\0ac7a354d21fb4d93efb0f4033e542f51af80f77095af13dc4e9086d69b65eff
Java version: 21.0.12, vendor: IBM Corporation, runtime: C:\hostedtoolcache\windows\Java_Adopt-OpenJ9_jdk\21.0.12-8.0.LTS\x64

[elharo]

Failed on https://github.com/apache/maven-compiler-plugin/actions/runs/33213550640/job/98995204562?pr=1118

[elharo]

Even if it's only flaky, I'd still ask if we actually need this one for some reason. Maven generally doesn't tie that tightly to the idiosyncracies of JDKs that we really need to test on multiple vendors.

[elharo]

and failed again on https://github.com/apache/maven-compiler-plugin/actions/runs/33251034368/job/99097486604?pr=1120

[slawekjaranowski]

ok, similar is on 3.x branch

[slawekjaranowski]

Legacy AdoptOpenJDK distributions were removed. Use temurin instead of adopt or adopt-hotspot, and semeru instead of adopt-openj9.

https://github.com/actions/setup-java#supported-distributions

[github-actions]

@elharo Please assign appropriate label to PR according to the type of change.