Chore: Modernize docs configuration for Python 3.12+

Chore: Modernize docs configuration for Python 3.12+

Remove broken/outdated Python modules that are incompatible with modern
Python versions. Update .readthedocs.yaml to use ubuntu-24.04 and Python
3.13. Modernize tox.ini (use allowlist_externals, pin basepython to
3.12, use local constraints instead of remote URLs that now 404).
Refresh docs/conf.py and update branch references to latest/master.
Add etc/upper-constraints.onap.txt from the doc repo.

Removed modules: sphinxcontrib-swaggerdoc, sphinxcontrib-spelling.

Issue-ID: CIMAN-33
Change-Id: Ibacba94f70dec8fae6ad15790b82037ca3974a3d
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>