PBS-style date-keyed releases + Node 24 actions

[FeodorFitsner]

Summary

Restructure python-build's release workflow to mirror what
astral-sh/python-build-standalone
does: a single GitHub release per date, containing every per-platform
tarball from every Python version in one place. Tarball filenames carry
the full Python version (3.14.6) so multiple patches of the same
minor can coexist in one release.

Why move off one-release-per-minor

• Re-cutting a release for a build fix (the Python.app strip in #N is
  the immediate example) had to clobber an existing v3.14 tag, which
  breaks reproducibility for any downstream that pinned to it.
• Several platform fixes recently had to wait for a coordinated "all
  minors get a fresh tarball" moment. Date tags decouple that — cut a
  new date once everything we care about builds, downstreams pick it up
  when they bump their PYTHON_BUILD_RELEASE pin.
• Symmetric with how flet-dev/serious-python's package_command
  already pins its astral-sh/python-build-standalone tarball by date.

Changes

build-python.yml

• workflow_dispatch gains a release_date input (YYYYMMDD).
• New publish-release job runs after the matrix, downloads every
  per-platform artifact, and publishes a single GitHub release tagged
  by that date.
• Publish only fires on workflow_dispatch with a non-empty
  release_date. Pushes still exercise the matrix (artifacts available
  for inspection) but never touch releases.

build-python-version.yml

• Per-version publish-release job removed (moved to parent).
• Artifact upload names now include the full Python version
  (python-darwin-3.14.6 instead of python-darwin-3.14) so the
  parent workflow can ingest multiple patches of the same minor
  without artifact-name collisions.
• Darwin packagers receive the full version directly.

Per-platform packagers (darwin/ios, android, linux, windows)

• Tarball / zip filenames switch from $PYTHON_VERSION_SHORT to the
  full $PYTHON_VERSION (e.g. python-macos-dart-3.14.6.tar.gz).
  Internal lookups that need the short version derive it inline.
• mobile-forge artifacts follow the same naming convention.

CI hygiene

• Bumped actions/checkout, actions/upload-artifact,
  actions/download-artifact, and softprops/action-gh-release to
  their latest majors (Node 24). Clears the 12 Node-20 deprecation
  warnings emitted by every matrix entry on the previous run.

Downstream impact (separate PR in flet-dev/serious-python)

After this merges and the first date-keyed release is cut, the five
plugins in serious-python need a follow-up to:

1. Read a new PYTHON_BUILD_RELEASE (YYYYMMDD) env var instead of
   composing URLs with v$PYTHON_VERSION.
2. Reference tarballs by full Python version in the URL.
3. Cache layout: $FLET_CACHE_DIR/python-build/<release>/<file>.

That migration is independent of this PR.

Test plan

• CI matrix on this branch builds green (every platform × version).
• On merge: trigger workflow_dispatch with release_date: <today>,
  confirm a single release is published with all 4 platforms × 3
  Python versions worth of tarballs.
• Confirm tarballs name themselves with the full version
  (python-macos-dart-3.14.6.tar.gz, not …-3.14.tar.gz).
• Confirm the softprops/action-gh-release@v3 publish step
  succeeds end-to-end.