MDL-43604 add support for future hacky MariaDB versions

This patch also shows the real server version without
normalisation on the environment page, this should
hopefully help people to identify MariaDB...
MDL-44191 hack around MySQL 5.6 inability to lower auto_increment
MDL-44339 treat unexpected debugging() calls in phpunit tests as errors
MDL-44608 Scheduled Tasks: Use $CFG->timezone if it is set to schedule tasks
MDL-44604 fix task cleanup in plugin uninstallation
MDL-44432 use new task scheduler in new logging plugins
MDL-42890 tool_log Default legacy reader should handle CRUD queries
MDL-42890 backup: Remove usage of log table in backup cron helper
MDL-44500 detect context-courseid inconsistencies in new events
MDL-44515 import latest PHPExcel 1.8.0
MDL-44516 import htmlpurifier 4.6.0
MDL-44521 import TCPDF 6.0.062
MDL-44522 import markdown 1.4.0
MDL-44527 import latest certificate bundle
MDL-44599 undeprecate SITEID

There is no way we could implement multitenant now,
it means there is no reason to deprecate SITEID.
MDL-44624 fix false failures in DML tests
MDL-44609 add component column to the scheduled task table
MDL-44585 grade_import: Restrict the accepted types in the gradebook import
MDL-44502 filelib: Add mimetype for Maple worksheets
MDL-44505 mod_forum: Make mailing subject a configurable language string