minor #18475 [Form] Document a deprecation related to model_timezone (javiereguiluz)

This PR was merged into the 6.4 branch.

Discussion
----------

[Form] Document a deprecation related to `model_timezone`

Fixes #18463.

Commits
-------

aa39730 [Form] Document a deprecation related to model_timezone

Author: OskarStark

reference/forms/types/options/model_timezone.rst.inc

@@ -6,4 +6,10 @@
 Timezone that the input data is stored in. This must be one of the
 `PHP supported timezones`_.
 
+.. deprecated:: 6.4
+
+    Starting from Symfony 6.4, it's deprecated to pass ``DateTime`` or ``DateTimeImmutable``
+    values to this form field with a different timezone than the one configured with
+    the ``model_timezone`` option.
+
 .. _`PHP supported timezones`: https://www.php.net/manual/en/timezones.php