3.7 (min PHP 7.1) requires 3.2 (first version supporting PHP 7.1)

This just deletes all the upgrade steps previous to 3.2.0. Some
small adjustments, like adding missing MOODLE_INTERNAL or tweaking
globals can also be applied when needed.

Also includes an upgrade step to prevent upgrading from any
version < 2016120500 (v3.2.0) as anti-cheating measure.

Next commit will get rid of/deprecate all the upgradelib functions
not used anymore in codebase. (note there isn't any this time).