This just deletes all the upgrade steps previous to 3.6.0. Some
small adjustments, like tweaking globals can also be applied
when needed.

Also includes an upgrade step to prevent upgrading from any
version < 2018120300 (v3.6.0) as anti-cheating measure.

Note that in this case, there wasn't any case of upgradelib
functions being used, hence we haven't to deprecate/remove
anything in codebase. When there is such a need, that is done
in separate commits (one for each function) and documented here.

See MDL-65809 commits for an example removing/deprecating a
good number of functions.

- PHP 7.3 required.
- Moodle 3.6 required.
- SQL*Server 2017 (aka, 14.0) required.
- Sodium PHP extension required.

This is a direct copy of the 3.10 branch to have
the changes under control in next commits.