Extend plugin processor message filtering and add [Log4j] prefixes

[DragonFSKY]

Fixes #4225.

This change:

• makes GraalVmProcessor respect the existing log4j.plugin.processor.minAllowedMessageKind annotation processor option;
• prefixes diagnostics emitted by PluginProcessor and GraalVmProcessor with [Log4j];
• preserves the default NOTE threshold and the existing severity filtering semantics;
• keeps plugin descriptor and GraalVM metadata generation independent from diagnostic filtering;
• updates the processor tests, plugin manual, and changelog.

The diagnostic prefix is an intentional text change. No generated file format or public API is changed.

Verification

• ./mvnw -pl log4j-core-test -am -Dtest=GraalVmProcessorTest,PluginProcessorPublicSetterTest -Dsurefire.failIfNoSpecifiedTests=false test (21 tests)
• ./mvnw -pl log4j-core,log4j-core-test spotless:check
• ./mvnw verify (41 reactor modules)
• git diff --check

Checklist

• Changes are based on the 2.x branch.
• The Maven verification succeeds.
• A changelog entry is included under src/changelog/.2.x.x.
• Tests are provided.

[vy]

LGTM.