Update harcoded links to MoodleDocs to point to correct wiki (2.0, 2.1 and 2.2)

While installing Moodle 1.9, link on "Current release information" for Release notes point to http://docs.moodle.org/en/Release_Notes, which redirects to 2.0 release notes.

Release link is hard-coded in code.

$releasenoteslink = get_string('releasenoteslink', 'admin', 'http://docs.moodle.org/en/Release_Notes');

It will be nice to grep all hard-coded strings and replace them with correct doc link, as part of this fix

Please check links while updating 1.9 to 2.0 and 2.0 to 2.1, links on "Current release information" for "Server Check" point to previous version docs.