Set maven.compiler.release unconditionally and drop the JDK profiles

[slachiewicz]

Related to #608. maven.compiler.release is now set from javaVersion unconditionally and the jdk8 and jdk9+ profiles are gone. Both consumers gate the option themselves: plexus-compiler only emits --release when the javac in use is 9 or newer, reading the version from the toolchain executable when forking, and the javadoc mojo checks its runtime version. That removes the toolchains flaw the old profile's comment admits (MNG-6943) and one moving part, with no behaviour change on JDK 9+.

The parent deliberately does not define maven.compiler.source or target next to release: maven-compiler-plugin 4.x forwards all three and javac rejects the combination, which is what #543 established and why #569 was withdrawn. This PR therefore does not fix the silent override in #608 on the 3.x plugin; the docs now say plainly that a project must override javaVersion, not source/target. A loud failure for the 3.x case would be an enforcer rule in a follow-up.

Verified with a child project against 40-SNAPSHOT on JDK 25, reading the javac command line:

plugin	child sets	result
3.16.0 on Maven 3.10.0-rc-1	nothing	--release 8
3.16.0	maven.compiler.target 11	--release 8, unchanged from 39
3.16.0	javaVersion 17	--release 17
4.0.0-beta-5 on Maven 4.0.0-SNAPSHOT	nothing	success
4.0.0-beta-5	maven.compiler.target 11	option --target cannot be used together with --release, unchanged from 39
4.0.0-beta-5	javaVersion 17	success

Behaviour change only on JDK 8 builds: this POM no longer sets source and target there, so the compiler plugin's own default of 1.8 applies unless the project sets them. Measured on Zulu 1.8.0_504 with Maven 3.9.16, child setting nothing:

parent	javac gets	class file
39	-source 8 -target 8	major 52
this PR	-source 1.8 -target 1.8	major 52

The compiler plugin's page says 3.13.0 "converts" release to source/target on JDK 8; MCOMPILER-582 added only ITs and docs, and its IT configures all three, so the conversion is the defaults coinciding. A child with javaVersion 7 cannot reach the difference: the parent's RequireJavaVersion rule rejects JDK 8 for range [7,) on 39 as well. The docs module builds.

This change was created with AI assistance.

[slachiewicz]

Branch rewritten. The previous version derived release from target and set all three properties, which fails on maven-compiler-plugin 4.x (see the table that was in the description). This version only sets maven.compiler.release from javaVersion and removes the two JDK profiles; the description is replaced accordingly.

This comment was created with AI assistance.