feat: prepare release java v2.20.0

[github-actions]

Automated release preparation for java-v2.20.0.

Upstream submodule reference

• Release tag: layer-javaagent/0.21.0
• Commit: faa4ca4671428134849ead9d395d9c0951a9ea00

Changes in this PR

• java/version.txt — records the current release version and upstream release tag
• java/layer-data.sh — updates the layer version and release branch link
• java/sample-apps/template.yaml — updates the sample layer version
• java/README.md — updates the release version in the title
• README.md — updates the latest layer link and component versions detected from the pinned submodule
• opentelemetry-lambda — pins the selected upstream release tag and commit
• changelog/java-v2.20.0.md — adds the release changelog fragment

Pre-merge checklist

• Replace the TODO in changelog/java-v2.20.0.md with the release changes
• Verify java/version.txt metadata
• Verify java/layer-data.sh and java/sample-apps/template.yaml
• Verify the version in the title of java/README.md
• Verify README.md component versions and release link
• Verify opentelemetry-lambda points to the upstream tag and commit listed above
• Approve this PR

What happens automatically after merge

1. Changelog assembled — the fragment is inserted into CHANGELOG.md and deleted
2. Tag created — java-v2.20.0 is created from the assembled changelog commit
3. Release build dispatched — the matching language build workflow is started at the new tag
4. Pre-release created — artifacts are built, Lambda layers are published, and a GitHub pre-release is created
5. Manual promotion — review and promote the pre-release from the GitHub Releases page