[!!!][BUGFIX] Clear Language Cache Configuration broken

Rename BE user TS option
options.clearCache.clearRTECache
to
options.clearCache.clearLangCache
to determine whether the language cache clear item should be shown.

Fixes: #39103
Releases: 6.1

Change-Id: I701dc9a879de4f3681f3fb441545eaa6b3e08e3e
Reviewed-on: https://review.typo3.org/12912
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn