[ticket/12311] Use valid composer schema - license - for extensions

http://tracker.phpbb.com/browse/PHPBB3-12311

PHPBB3-12311
[ticket/12311] Use British-English spelling of licence in lang file

PHPBB3-12311
Merge remote-tracking branch 'vsephpbb/ticket/12311' into develop-ascraeus

* vsephpbb/ticket/12311:
  [ticket/12311] Use British-English spelling of licence in lang file
  [ticket/12311] Use valid composer schema - license - for extensions