[v1.x] docs: pin mkdocs<2

MkDocs 2.0 is a ground-up rewrite that drops the plugin system, so it is
incompatible with mkdocs-material and the rest of the docs plugin stack.
Mirror the bound from main (#3072) on the direct dependency so a future
re-lock on this branch cannot resolve 2.0. The locked versions are
unchanged; today the branch is protected only transitively by
mkdocs-material 9.6.x's own mkdocs~=1.6 requirement.

Author: maxisbey

pyproject.toml

@@ -69,7 +69,9 @@ dev = [
     "coverage[toml]==7.10.7",
 ]
 docs = [
-    "mkdocs>=1.6.1",
+    # MkDocs 2.0 is a ground-up rewrite (no plugin system) that is incompatible
+    # with mkdocs-material and every plugin below; stay on the 1.x line.
+    "mkdocs>=1.6.1,<2",
     "mkdocs-glightbox>=0.4.0",
     "mkdocs-material[imaging]>=9.6.19",
     "mkdocstrings-python>=1.12.2",