Build using newest JDK, still producing JDK 11 bytecode

[krzema12]

See what was done in krzema12/snakeyaml-engine-kmp#255.

Using the newest JDK makes use of latest advancements included there. Such SDK can produce bytecode runnavle by other JDKs.

[krzema12]

It would also let us upgrade to Gradle 9: #2041.