MDL-71747 upgrade: remove all the < 3.9.0 upgrade steps

MDL-71747 upgrade: remove all the < 3.9.0 upgrade steps

This just deletes all the upgrade steps previous to 3.9.0. Some
small adjustments, like tweaking globals can also be applied
when needed.

Also includes an upgrade step to prevent upgrading from any
version < 2020061500 (v3.9.0) as anti-cheating measure.

The next commits will proceed to deprecate / remove functions
that were exclusively (usually belonging to upgradelib) being
used by those, now removed, upgrade steps. This is the list
of code to remove and document in upgrade.txt files:

  - upgrade_analytics_fix_contextids_defaults()
  - upgrade_convert_hub_config_site_param_names()
  - upgrade_rename_prediction_actions_useful_incorrectly_flagged()
  - \mod_forum\task\refresh_forum_post_counts adhoc task.

And these is the code that has NOT been removed
because it may be needed later (no matter there aren't uses now):

  - \core_search\manager::clean_up_non_existing_area(), used by
  - \core\task\clean_up_deleted_search_area_task adhoc task