[FEATURE] Migration wizard for RTE magic images missing

All magic images of the RTE are still residing in uploads/
with a name like RTEmagicC_*.

The update wizard moves the files to fileadmin/_migrated_/RTE/
and adds them to the file index.

Resolves: #46614
Releases: 6.1
Change-Id: Ie5fa6bdf84e994e9e1ac3c804ce4eecdce072c86
Reviewed-on: https://review.typo3.org/19232
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack