[TASK] Deprecate legacy form templates

Using the legacy form template / partial variants residing in
:file:`EXT:form/Resources/Private/Frontend/Templates` and
:file:`EXT:form/Resources/Private/Frontend/Partials`
is deprecated. The legacy templates will be removed in v13.

Resolves: #95456
Releases: main
Change-Id: Ib1346065d69a12d6664758a3c9bedf7efa3bd834
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71423


Tested-by: core-ci <typo3@b13.com>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: waldhacker <hello@waldhacker.dev>