CI/documentation: constrain pygments to avoid latest version (2.13.0) (#32163)

All PRs are failing the docs build on account of an error with
pygments. These errors coincide with a new release of pygments
(2.13.0) and restricting to < 2.13 allows the doc tests to pass,
so this commit enforces that constraint for the docs build.

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

lib/spack/docs/requirements.txt

@@ -8,3 +8,4 @@ python-levenshtein
 # Restrict to docutils <0.17 to workaround a list rendering issue in sphinx.
 # https://stackoverflow.com/questions/67542699
 docutils <0.17
+pygments <2.13