chore: prepare release workflow for codegen 3.0 adjustments

[ewaostrowska]

This PR fixes the release-prepare flow regressions found in the review and aligns it with expected 3.0.0 release behavior.

What Changed

1. Enforced branch scope to 3.0.0 only.

• Added a hard guard in CI/release/prepare-codegen-release.sh to fail unless current branch is 3.0.0.
• Added workflow-level gating in .github/workflows/prepare-release-3.yml:
  • prepare job runs only when github.ref_name == '3.0.0'.
  • non-3.0.0 runs fail fast with explicit message.

2. Fixed version updates missed by prepare flow.

• pom.docker.xml root is now updated to the target release version.
• samples/meta-codegen/pom.xml is now updated to the target codegen release version.
• Implemented in CI/release/update-codegen-release-files.py.

3. Switched from repo file release notes to GitHub draft release notes.

• Removed creation of docs/release-notes/v.md in repo.
• Release notes are now generated in a temporary path (${RUNNER_TEMP}) and exposed as workflow output.
• Added workflow step using ncipollo/release-action to create/update a draft GitHub Release with those notes.