[TASK] Add feature switch "unifiedPageTranslationHandling"

The feature switch "unifiedPageTranslationHandling" is active for all new
installations, but not active for existing installations.

It does the following when active:
- All DB schema migrations decide to drop "pages_language_overlay"
- TCA migration no longer throws a deprecation info (but still unsets "pages_language_overlay")

Once the Update Wizard for migrating pages_language_overlay records is done,
the feature is enabled.

Resolves: #83711
Releases: master
Change-Id: I478c8d5d745309889fa38b44b5eaab7226afdfc4
Reviewed-on: https://review.typo3.org/55446


Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Tobi Kretschmann <tobi@tobishome.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>