[TASK] Add missing breaking rst for form setup mixins removal

Adds the forgotten breaking rst for the ticket
https://review.typo3.org/c/Packages/TYPO3.CMS/+/70528/

Resolves: #94911
Releases: master
Change-Id: I1a0e9c1b074b377febd655cc96a1a1033b56fc35
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70652


Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: waldhacker <hello@waldhacker.dev>

typo3/sysext/core/Documentation/Changelog/master/Breaking-70528-DeprecatedFormMixinsRemoved.rst

+.. include:: ../../Includes.txt
+
+=================================================
+Breaking: #70528 - Deprecated form mixins removed
+=================================================
+
+See :issue:`70528`
+
+Description
+===========
+
+The deprecated ext:form setup mixins from :yaml:`TYPO3.CMS.Form.mixins.*` have been removed.
+
+Impact
+======
+
+Form setup inheriting mixins from :yaml:`TYPO3.CMS.Form.mixins.*` will not work properly anymore.
+
+Affected Installations
+======================
+
+All installations using the deprecated form setup mixins are affected.
+
+Migration
+=========
+
+Embed the essential parts from :yaml:`TYPO3.CMS.Form.mixins.*` or migrate them to custom mixins.
+
+.. index:: Backend, Frontend, NotScanned, ext:form