Fix issues with PLUGIN_FILTERS_CFG config handling (#45994)

* Ensure that the value of PLUGIN_FILTERS_CFG is treated as type=path, and that we use the standard section of 'defaults' instead of 'default'

* deprecate the default section

* Don't add version_added for the corrected section

lib/ansible/config/base.yml

@@ -1549,6 +1549,13 @@ PLUGIN_FILTERS_CFG:
   ini:
   - key: plugin_filters_cfg
     section: default
+    deprecated:
+      why: Specifying "plugin_filters_cfg" under the "default" section is deprecated
+      version: "2.12"
+      alternatives: the "defaults" section instead
+  - key: plugin_filters_cfg
+    section: defaults
+  type: path
 RETRY_FILES_ENABLED:
   name: Retry files
   default: True