[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
(cherry picked from commit 57284b29fce74160f23ffc84c6f3f28f92f8a93d)
Reviewed-on: https://review.typo3.org/29496