[TASK] Deprecate TSFE->sys_language_isocode

Due to the introduction of Site Handling, the public property
`$TSFE->sys_language_isocode` is not necessary anymore,
as this value is used outside of TSFE fully, and not needed as
public property to hold the state.

It is marked as deprecated.

Resolves: #88559
Releases: master
Change-Id: If02eb5b342fec4f83a9585dbc29fc13c8adda88e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61002


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>