[TASK] Deprecate switchable controller actions

The usage of switchable controller actions, both via
flexforms and typoscript, is deprecated and will be
removed in one the next major versions of TYPO3,
probably version 11.0 or 12.0.

Switchable controller actions allowed to override the
php plugin configuration and to create god plugins, i.e.
plugins that can be set into multiple different modes
and therefore take care of all possible use cases.

Every plugin should serve a single purpose, therefore
the usage of switchable controller actions is an anti
pattern which will be removed.

The switchable controller action mechanic will be
removed without replacement which means, that there
is no migration path to a similar feature.

Instead, extension authors need to create multiple,
dedicated plugins for different use cases.

Releases: master
Resolves: #89463
Change-Id: I41afac9303205f97f390f208803908177e00cda5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61511


Tested-by: Richard Haeser <richard@maxserv.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Susanne Moog <look@susi.dev>