docs: try upgrading mkdocs-material again, upgrade MkDocs

squidfunk/mkdocs-material#7160 is supposed to
be fixed now. I will double-check that the version switcher still works
after this commit. This only affects the version switcher from
"prerelease" to other versions.. The functionality is a bit brittle, so
it's hard to test locally before merging this.

We do want to upgrade, since this allows us (and seems to also require
us) to upgrade to MkDocs 1.6, which has a nice feature of disallowing
links to broken document section (beyond just checking for links to
non-existent files).

I carefully avoided unnecessary upgrades in this commit, but we
should also run `poetry update` shortly, once we check this
works properly. Also, the feature of MkDocs 1.6 I mentioned needs
enabling.

Finally, I plan to change version specifiers from Poetry's `^5.6.22`
syntax to more plain inequalities, since I keep forgetting what the
former syntax means. I started with this commit.

(I was surprised MkDocs was allowed to be upgraded to `1.6` with the
`^1.5.2` spec.  I now suspect that what I previously thought it should
mean is expressed as `~1.5.2`, but inequalities are clearer).

pyproject.toml

@@ -13,7 +13,7 @@ python = "^3.8"
 [tool.poetry.group.dev.dependencies]
 # These can be updated with `poetry add`.
 mkdocs = "^1.5.2"
-mkdocs-material = "=9.5.4" # Until https://github.com/squidfunk/mkdocs-material/issues/7160 is fixed
+mkdocs-material = ">=9.5.22,<9.6"
 # (Py)Markdown extensions
 mdx-truly-sane-lists = "^1.3"
 mdx-breakless-lists = "^1.0.1"