Remove docs python version pin

By default, opendev now runs on Ubuntu noble nodes which have
Python 3.12 and not 3.11.  Remove the version pin which forced
3.11 in the docs jobs so that they will now run on 3.12.

Also, update the libffi8 bindep selector.

Change-Id: I6ff8e1d0738d6ca4b654739fc6cdff8d89d732fe

.zuul.yaml

@@ -256,10 +256,7 @@
       jobs:
         - opendev-buildset-registry
         - nodepool-build-image
-        - zuul-nox-docs:
-            vars:
-              nox_force_python: "3.11"
-              python_version: "3.11"
+        - zuul-nox-docs
         - nox-linters:
             nodeset: ubuntu-jammy
         - nodepool-nox-py311
@@ -272,10 +269,7 @@
     gate:
       jobs:
         - nodepool-upload-image
-        - zuul-nox-docs:
-            vars:
-              nox_force_python: "3.11"
-              python_version: "3.11"
+        - zuul-nox-docs
         - nox-linters:
             nodeset: ubuntu-jammy
         - nodepool-nox-py311

bindep.txt

@@ -5,9 +5,9 @@ gcc [compile test]
 libc6-dev [compile test platform:dpkg]
 libffi-dev [compile test platform:dpkg platform:apk]
 libffi [platform:apk]
-libffi8 [platform:ubuntu-jammy platform:debian-bookworm]
+libffi8 [platform:ubuntu-jammy platform:ubuntu-noble platform:debian-bookworm]
 libffi7 [platform:ubuntu-focal platform:debian-bullseye]
-libffi6 [platform:dpkg !platform:ubuntu-focal !platform:ubuntu-jammy !platform:debian-bullseye !platform:debian-bookworm]
+libffi6 [platform:dpkg !platform:ubuntu-focal !platform:ubuntu-jammy !platform:ubuntu-noble !platform:debian-bullseye !platform:debian-bookworm]
 libffi-devel [compile test platform:rpm]
 libressl-dev [compile test platform:apk]
 libssl-dev [compile test platform:dpkg]