[TASK] Move special sys_refindex DB index to ext:form

"KEY lookup_string (ref_string(191))" is only used in
a query performed by ext:form. This extension is a bit
specific with using sys_refindex and is the only one
that queries using "where ref_string=...". To reduce
index size of sys_refindex a bit, that index is now
moved to ext:form to be added only if that extension
is loaded.

Resolves: #104368
Releases: main
Change-Id: I3315633590089bb7f7997865fbba34ecca9c250a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85228


Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>