[BUGFIX] Move legacy indexed_search plugin registration

When moved to EXT:compatibility7 the pi-based Indexed Search
plugin registration was moved to "Configuration/TCA" but
should go into "Configuration/TCA/Overrides".

Resolves: #79299
Releases: master
Change-Id: Ic9abec2a967b08514a786bc2ee62ce65024a1e32
Reviewed-on: https://review.typo3.org/51288


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>