[TASK] Deprecate backwards-compatibility TYPO3 language keys

Before TYPO3 v4.0, TYPO3 had inconsistencies
in its language keys, such as "ja" (= Japan)
instead of "jp" (= Japanese), which was still
available.

However, the mapping was only used for
detecting the language from the user agent,
for the backend login screen, and not used
since various versions.

TYPO3 since at least 6.0 only delivers
the correct language keys and the "old"
language keys can now safely be removed,
thus they are deprecated.

The getter method Locales->getIsoMapping()
is marked as deprecated.

Resolves: #99531
Releases: main
Change-Id: Ie86237fd0ac4bad460960f31e3bd3971d021de16
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77373


Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>