[TASK] Add index to l10n_source field

The database field which is created based on TCA setting
translationSource, now automatically gets an index.
This improves the performace in (for example) the
migration from pages_language_overlay to pages.

Resolves: #89853
Releases: master, 9.5
Change-Id: I23ba8254158250249422b9fb0d20975372a11dbd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62534


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Richard Haeser <richard@maxserv.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Jonas Eberle <flightvision@googlemail.com>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Steffen Frese <steffenf14@gmail.com>
Tested-by: Tobi Kretschmann <tobi@tobishome.de>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jonas Eberle <flightvision@googlemail.com>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Steffen Frese <steffenf14@gmail.com>
Reviewed-by: Tobi Kretschmann <tobi@tobishome.de>