doc: Upgrade and unpin some python versions

https://access.redhat.com/security/cve/CVE-2022-34749

https://bugzilla.redhat.com/show_bug.cgi?id=2255448

Fixes https://github.com/ceph/ceph/pull/44222

Signed-off-by: David Galloway <david.galloway@ibm.com>

.readthedocs.yml

@@ -7,7 +7,7 @@ formats: []
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.8"
+    python: "3.9"
   apt_packages:
     - ditaa
     - graphviz

admin/doc-requirements.txt

@@ -1,4 +1,5 @@
-Sphinx == 5.0.2
+# RTD theme does not work with >= 7 https://github.com/readthedocs/readthedocs.org/issues/10279
+Sphinx < 7
 git+https://github.com/ceph/sphinx-ditaa.git@py3#egg=sphinx-ditaa
 git+https://github.com/vlasovskikh/funcparserlib.git
 breathe >= 4.20.0,!=4.33
@@ -10,11 +11,13 @@ pcpp
 prettytable
 sphinx-autodoc-typehints == 1.18.3
 sphinx-prompt
-sphinx_rtd_theme == 1.0.0
+sphinx_rtd_theme
 Sphinx-Substitution-Extensions
 typed-ast
 sphinxcontrib-mermaid
 sphinxcontrib-openapi
 sphinxcontrib-seqdiag
-mistune < 2.0.0
+# m2r2 replaces mistune https://github.com/CrossNox/m2r2?tab=readme-ov-file#m2r-the-original
+m2r2
 natsort
+docutils < 0.20