deprecate invalid variable names in inventory (#85377)

* deprecate invalid variable names in inventory

* was previously a hard error for `Host`
* added missing check to `Group`
* swapped blanket Python keyword prohibition with Jinja singleton and `not` check

Co-authored-by: Matt Clay <matt@mystile.com>

* fix invalid variable name test

---------

Co-authored-by: Matt Clay <matt@mystile.com>
(cherry picked from commit 3c52b14c9e)

changelogs/fragments/templates_types_datatagging.yml

@@ -92,7 +92,6 @@ breaking_changes:
   - templating - Access to ``_`` prefixed attributes and methods, and methods with known side effects, is no longer permitted.
     In cases where a matching mapping key is present, the associated value will be returned instead of an error. 
     This increases template environment isolation and ensures more consistent behavior between the ``.`` and ``[]`` operators.
-  - inventory - Invalid variable names provided by inventories result in an inventory parse failure. This behavior is now consistent with other variable name usages throughout Ansible.
   - internals - The ``ansible.utils.native_jinja`` Python module has been removed.
   - internals - The ``AnsibleLoader`` and ``AnsibleDumper`` classes for working with YAML are now factory functions and cannot be extended.
   - public API - The ``ansible.vars.fact_cache.FactCache`` wrapper has been removed.

changelogs/fragments/variable_names.yml

@@ -0,0 +1,7 @@
+bugfixes:
+  - variables - Added Jinja scalar singletons (``true``, ``false``, ``none``) to invalid Ansible variable name detection.
+    Previously, variables with these names could be assigned without error, but could not be resolved.
+minor_changes:
+  - variables - Removed restriction on usage of most Python keywords as Ansible variable names.
+deprecated_features:
+  - inventory plugins - Setting invalid Ansible variable names in inventory plugins is deprecated.