[BUGFIX] Invalid RSS language definition

The default language for RSS feed 'en_GB' does
not comply to ISO-639.

Change it to 'en-gb'.

Resolves: #57970
Change-Id: Ic2117acf2ee3c4849bb272d3733dce2f39bab0bc
Reviewed-on: https://review.typo3.org/29494
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer