[!!!][TASK] Move indexed search pi1 to EXT:compatibility7

For further cleanups a new system extension is introduced
that will be moved in to TER before the release of 8 LTS.

Move indexed search pi-based plugin to compatibility7.

Resolves: #75349
Releases: master
Change-Id: I699dbb4b987df92e7dc81d8b5c65ff05b2e2a958
Reviewed-on: https://review.typo3.org/47469


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>