Pin setuptools<81 and sphinx<8 for sphinxcontrib-needs

Pin setuptools<81 and sphinx<8 for sphinxcontrib-needs

sphinxcontrib-needs 0.7.9 uses pkg_resources (removed in
setuptools>=81) and sphinx.util.status_iterator (removed in
Sphinx 8). Pin both to unblock the RTD build.

Also add pins to etc/requirements.txt so that the root tox.ini
builds (needs, docs-linkcheck) are equally protected.

Issue-ID: CIMAN-33
Co-Authored-By: Claude <noreply@anthropic.com>
Change-Id: I18a954ad30dff923620934d78e1f947ccf2b6405
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>