[TASK] Remove "ISO 639-1" option from site configuration

Due to the introduction of the unified
Locale class, the SiteLanguage now
resolves the two-letter-iso code from the
locale if it wasn't set before.

It is thus removed from the GUI when editing
or creating site languages, and also automatically
populated as fallback, however it is recommended
to fill the locale properly and the method
is deprecated.

Resolves: #99905
Releases: main
Change-Id: Ib98a119b97ae81c96e50fda9835adc36ac41eb37
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77597


Tested-by: core-ci <typo3@b13.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Georg Ringer <georg.ringer@gmail.com>