Help: Update Sphinx versionadded directives for 3.28 release

Run the script:

    Utilities/Sphinx/update_versions.py --since v3.27.0 --overwrite

Manually remove directives added to new documentation of existing
environment variables.

Help/policy/CMP0153.rst

@@ -1,6 +1,8 @@
 CMP0153
 -------
 
+.. versionadded:: 3.28
+
 The :command:`exec_program` command should not be called.
 
 This command has long been superseded by the :command:`execute_process`