Update version constraints in requirements.txt files

[Mandrenkov]

Context:

Building the XST website using the current requirements.txt files fails due to incompatible packages:

$ make docs
make -C doc html
Running Sphinx v4.5.0

Sphinx version error:
The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.
make[1]: *** [html] Error 2
make: *** [docs] Error 2

Description of the Change:

1. Pinned some implicit requirements in doc/requirements.txt (to match the PennyLane Sphinx Theme).
2. Bumped the pinned version of pillow to resolve some vulnerability alerts.
3. Bumped the pinned version of black to resolve some vulnerability alerts.
4. Updated the Python version used in the Formatting Check workflow to reflect (3).

Benefits:

1. It is now possible to build the test documentation website locally using the provided requirements.txt files.
2. There are no outstanding vulnerability alerts for the Xanadu Sphinx Theme.

Possible Drawbacks:
None.

Related GitHub Issues:
None.