1.21.7 release notes and version update

Change-Id: Ibaf890a55e5beb8827b36fc2c05b12b3f4f6d624
Merge branch 'REL1_21' of https://gerrit.wikimedia.org/r/p/mediawiki/core into REL1_21
Merge branch 'REL1_21' into REL1_21_custom