ci: add RC promotion command

[Haricharanpanjwani]

Part of #715.
Closes #717.

Summary

• add a new promote subcommand to scripts/apache_release.py to promote a voted RC from Apache SVN dist/dev to dist/release
• validate the expected source archive, sdist, wheel, and matching .asc / .sha512 files before promotion
• remove the current dist/release contents before copying in the promoted artifacts, matching ASF "latest only" release policy
• print the final twine upload command for the promoted release artifacts
• support --dry-run so the command shows the intended release cleanup, copy, and commit steps without committing
• support incubating -> TLP path transitions through --dev-svn-root and --release-svn-root overrides
• document the new promotion flow in scripts/README.md
• add focused pytest coverage for RC parsing, artifact validation, rename behavior, PyPI command rendering, and dry-run orchestration

How I tested this

• ran python3 -m pytest tests/test_apache_release.py
• ran python3 -m py_compile scripts/apache_release.py tests/test_apache_release.py
• ran python3 scripts/apache_release.py promote --help

Notes

• the promotion flow is implemented in scripts/apache_release.py rather than as a separate script so it stays alongside the existing release build, upload, and verification commands
• the command defaults to incubator SVN paths today, but accepts --dev-svn-root and --release-svn-root so it can handle the post-incubation TLP path change without code changes
• this PR covers the issue acceptance criteria by adding a single promotion command, validating the required artifacts before promotion, supporting incubator/TLP SVN root overrides, and providing a non-committing dry-run mode
• I did not run a full end-to-end promote --dry-run against live SVN in this environment; the command still requires SVN access to validate the real RC and release paths before planning the promotion steps

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal
• Any change in functionality is tested
• Project documentation has been updated for the new command

[Haricharanpanjwani]

@skrawcz @andreahlert can you review this PR?