Define more conventions for all GradleX projects

[jjohannes]

To Discuss

1. Rename: internal.plugin-publish-conventions -> internal-build-conventions (agreed ✅)
2. Switch to // SPDX-License-Identifier: Apache-2.0 license header (agreed ✅)
3. Sample tests running with "Vintage Runner". Pick one setup:
   • Test tags no-cross-version: org.gradlex.testing.NoCrossVersion inside test (several repos)
   • Separate test suite testSamples that does not run cross version (maven-plugin-development) (agreed ✅)
4. Test sample optional input dirs:
   • For examples that users may look at: samples (in root) - agreed ✅
   • For samples that contain doc snippets. Pick one:
     • src/docs/samples (build-parameters)
     • src/docs/snippets (maven-plugin-development) - agreed ✅

Follow ups

• @NullMarked: don't use package-info.java - Add check that forbids package-info.java files #65
• Replace Exemplar with "normal" tests - Replace Exemplar with normal TestKit tests #63
• As future issue - add checks to enforce setup - Add check for a consistent project setup #66
• Asciidoctor? - Evaluate if we can remove the dependency to asciidoctor plugin #64

Repository switch PRs

• Migrate build to shared convention plugins setup extra-java-module-info#205
• Migrate build to shared convention plugins setup jvm-dependency-conflict-resolution#323
• Migrate build to shared convention plugins setup reproducible-builds#57
• Migrate build to shared convention plugins setup java-module-testing#138
• Migrate build to shared convention plugins setup java-module-packaging#79
• Migrate build to shared convention plugins setup java-module-dependencies#252
• Migrate build to shared convention plugins setup build-parameters#260
• Migrate build to shared convention plugins setup java-module-dependencies-maven-extension#14
• Migrate build to shared convention plugins setup gradle-module-metadata-maven-plugin#59 (no verification yet)
• Migrate build to shared convention plugins setup maven-plugin-development#372 (no verification yet)