Add validation for Plexus-based plugin dependency injection

[slawekjaranowski]

What changed

• Added a new validation that detects outdated dependency injection usage in plugins.
• Added a clear warning for plugin developers when this pattern is found.
• Pointed users to the recommended modern injection approach.

based on: 2a8b6b2

[gnodet]

Clean cherry-pick of the already-merged #12577 to the maven-3.10.x maintenance branch. The validator logic is identical to master; the test was properly adapted for the 3.10.x MojoDescriptor API (real instance via ComponentDescriptor.addRequirement rather than mocking). Adding mockito-junit-jupiter as a separate dependency is correct for 3.10.x, which doesn't use mockito-bom like master.

Two minor non-blocking nits (same as noted on #12649):

• pluginValidationManager field is protected but this is a concrete, package-private, final class with no subclasses — should be private
• Missing @since 3.10.0 Javadoc tag (other validators in the same package include @since)

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Claude Code on behalf of gnodet