[Platform-Dependencies] Remove Sphinx constraint and fix docs build

[UsamaSadiq]

Description

• Remove Sphinx constraint and fix the failing docs build occurring due to new sphinx and doc-utils version.
• Also remove the sphinxcontrib-openapi[markdown] constraint from development.in file which was present because of Python3.6 requirement.

Error Stack Trace

WARNING: autodoc: failed to import module 'urls' from module 'cms'; the following exception was raised:
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/site-packages/sphinx/ext/autodoc/importer.py", line 58, in import_module
    return importlib.import_module(modname)
  File "/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 843, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/runner/work/edx-platform/edx-platform/cms/urls.py", line 188, in <module>
    if not settings.DISABLE_DEPRECATED_SIGNIN_URL:
  File "/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/site-packages/django/conf/__init__.py", line 83, in __getattr__
    val = getattr(self._wrapped, name)
AttributeError: 'Settings' object has no attribute 'DISABLE_DEPRECATED_SIGNIN_URL'

[UsamaSadiq]

Investigated the issue and created PR openedx/edx-platform#31152 to fix this issue.