Add Moodle 2.1.0 upgrade line to all the upgrade.php scripts

In order to have it properly detected for the future it would be great to add to all the upgrade.php scripts some lines like these:

// Moodle v2.1.0 release upgrade line

// Put any upgrade step following this

exactly before the "return true;" present in all the scripts.

I think it's ok to do that both in the 21_STABLE and master branches, so they will allow quickly find where 2.1.0 started and act once we decide the Moodle 2.2 requirements.

Command to detect all the upgrade.php files not having those lines:

find . -name upgrade.php | xargs grep -vl "Moodle v2.1.0 release upgrade line"

Ciao