Build against Maven 4.0.0-rc-6

[slachiewicz]

Moves mavenVersion to the current RC and pins the same version explicitly in the Verify workflow, so CI no longer depends on the maven4-version default in maven-gh-actions-shared.

The version bump alone made JarMojoTest fail:

JarMojoTest.jarTestEnvironment:48 expected: <foo> but was: <myGroupId>

rc-6 relocated org.apache.maven.api.plugin.testing to org.apache.maven.testing.plugin. The old names survive as deprecated shims, so the test still compiled — but MojoExtension's annotation lookup only recognises the new annotation types. With the old ones the @InjectMojo attributes come back null, the @Basedir pom is never read, and the mojo is handed MojoExtension's built-in default model. Pointing the imports at the new package restores it; no production code changes.

Worth flagging generally: this is a silent behavioural break, not a compile error, so any plugin still importing the deprecated package will keep building and quietly stop reading its test poms. Same relocation as apache/maven-compiler-plugin#1104.

Part of apache/maven#12676. Verified green on a fork before opening.

[slachiewicz]

Superseded by #573, which is already merged and makes exactly the same three changes — ff-maven/maven-matrix to rc-6, mavenVersion to rc-6, and the org.apache.maven.testing.plugin import relocation. Closing.