[devel] Bump antsibull-docs to 2.21.0; include list of all deprecated collections and modules/plugins (#2979)

* Bump antsibull-docs dependency constraint to 2.21.0.

* ci: refresh docs build dependencies

* Add list of all deprecations to all plugins index.

---------

Co-authored-by: Felix Fontein <[email protected]>
Co-authored-by: Ansible Documentation Bot <147556122+ansible-documentation-bot[bot]@users.noreply.github.com>

Author: ansible-documentation-bot[bot]

docs/docsite/rst/collections/all_plugins.rst

@@ -9,3 +9,4 @@ Indexes of all modules and plugins
    :glob:
 
    index_*
+   deprecations

tests/constraints.in

@@ -3,7 +3,7 @@
 # This constraint file also pins other versions for which there are known limitations.
 
 sphinx == 7.2.5
-antsibull-docs == 2.20.0  # currently approved version
+antsibull-docs == 2.21.0  # currently approved version
 
 sphinx-rtd-theme >= 2.0.0  # Fix 404 pages with new sphinx -- https://github.com/ansible/ansible-documentation/issues/678
 rstcheck < 6  # rstcheck 6.x has problem with rstcheck.core triggered by include files w/ sphinx directives https://github.com/rstcheck/rstcheck-core/issues/3

tests/requirements.txt

@@ -26,7 +26,7 @@ antsibull-changelog==0.34.0
     # via antsibull-docs
 antsibull-core==3.4.0
     # via antsibull-docs
-antsibull-docs==2.20.0
+antsibull-docs==2.21.0
     # via
     #   -c tests/constraints.in
     #   -r tests/requirements.in