[TASK] Document deprecation of legacy form templates

Resolves: #97916
Related: #95456
Releases: main
Change-Id: I5bf875d11fc91b687b9fba36c2d8c23f5eae1756
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78687


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>

typo3/sysext/form/Documentation/I/Config/proto/formElements/formElementTypes/Form/renderingOptions/templateVariant.rst

 .. include:: /Includes.rst.txt
+
 renderingOptions.templateVariant
 --------------------------------
 
 :aspect:`Option path`
-      prototypes.<prototypeIdentifier>.formElementsDefinition.Form.renderingOptions.templateVariant
+    prototypes.<prototypeIdentifier>.formElementsDefinition.Form.renderingOptions.templateVariant
 
 :aspect:`Data type`
-      array
+    array
 
 :aspect:`Needed by`
-      Frontend
+    Frontend
 
 :aspect:`Overwritable within form definition`
-      Yes
+    Yes
 
 :aspect:`form editor can write this property into the form definition (for prototype 'standard')`
-      No
+    No
 
 :aspect:`Mandatory`
-      No
+    No
 
 :aspect:`Default value (for prototype 'standard')`
+    ..  code-block:: yaml
 
-      .. code-block:: yaml
-
-         templateVariant: version1
+        templateVariant: version1
 
 :aspect:`Description`
-      Set this option to `version2` to use Bootstrap 5 compatible and
-      accessible templates.
+    Set this option to :yaml:`version2` to use Bootstrap 5 compatible and
+    accessible templates.
+
+    ..  deprecated:: 12.0
+        Using the legacy form template / partial variants residing in
+        :file:`EXT:form/Resources/Private/Frontend/Templates` and
+        :file:`EXT:form/Resources/Private/Frontend/Partials` ("version1") is
+        deprecated. The legacy templates will be removed in v13.
+
+        **Migration**: Set your form rendering option :yaml:`templateVariant`
+        within the form setup from :yaml:`version1` to :yaml:`version2` to use
+        the future default templates:
+
+        ..  code-block:: yaml
+
+            prototypes:
+              standard:
+                formElementsDefinition:
+                  Form:
+                    renderingOptions:
+                      templateVariant: version2
+
+        Adjust your templates / partials to make them compatible with the ones
+        stored in :file:`EXT:form/Resources/Private/FrontendVersion2`.