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.

Upgrade step that resets the 'Data modification API' (convert_data)
setting to its default value if this setting is currently configured
to use a file located within the $CFG->dataroot directory.

Prevents configuring the 'Data modification API' (convert_data) setting
to use files located within the $CFG->dataroot directory as it exposes
the site to security risks.