[CLEANUP] Use correct file extension for language files

Remove ".php" and replace with ".xlf"

Resolves: #72520
Releases: 7.6, master
Change-Id: I867f7e1911ccf194e6ab579f4f18d20b0554d57c
Reviewed-on: https://review.typo3.org/45562


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>