Help: Update Sphinx versionadded directives for 3.21 release

Run the script:

    Utilities/Sphinx/update_versions.py --since v3.20.0 --overwrite

Manually remove directives generated in documents for long-existing
features for which documentation was added.

Help/variable/CPACK_CUSTOM_INSTALL_VARIABLES.rst

@@ -1,6 +1,8 @@
 CPACK_CUSTOM_INSTALL_VARIABLES
 ------------------------------
 
+.. versionadded:: 3.21
+
 CPack variables (set via e.g. ``cpack -D``, ``CPackConfig.cmake`` or
 :variable:`CPACK_PROJECT_CONFIG_FILE` scripts) are not directly visible in
 installation scripts.  Instead, one can pass a list of ``varName=value``