[!!!][TASK] Deprecate the field select_key of tt_content

The field `select_key` is not used by the core and
is moved to the EXT:compatibility7.

Resolves: #77934
Releases: master
Change-Id: Ied7fa722c1bccb314fd02229da407613fbce7dfe
Reviewed-on: https://review.typo3.org/49934


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>