[TASK] Add xlf to [SYS][textfile_ext]

Add the xlf file extension to the list of textfile extensions in the
default configuration.

Resolves: #53136
Releases: 6.2
Change-Id: I2ea509c8a027bb21d77a59313cce4dfa9495c280
Reviewed-on: https://review.typo3.org/25008
Reviewed-by: Philipp Gampe
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer