Drop support for MediaWiki 1.21

As a consequence, wikis on MediaWiki 1.21 can no longer update to the
latest version of Translate. However, most of those wikis are running
MLEB and MLEB 2014.06 has already dropped support for 1.21.

According to WikiApiary, 20% of wikis running Translate are using 1.21,
33% are using <= 1.21. Among them, only one is using a Translate version
released in 2014.

Change-Id: If38ca794b0bfca1d3b3fe2f57ea9e2eb9a717639