[6.0] Initial update script clean up for 6.0 (joomla#43598)

* Remove 5.x update SQL scripts + add 6.0.0 dummy

* Clean up uninstallExtensions and adapt to 6.0.0

Clean up core extensions uninstallation and migration on update for Joomla 6 and remove obsolete migration functions.

* Init lists of deleted files and folders and renamed files

* Clean up postflight migrations and adapt to 6.0.0

Clean up postflight method for Joomla 6 and remove obsolete migration functions.

* Clean up updateAssets method

* Add files and folders to be deleted on update

- Add 5.x update SQL scripts
- Add files abd folder from PR joomla#42884
- Add files from PR joomla#42890

* Fix "From" version in comment

* Add example to comment in uninstallExtensions method

* Remove obsolete exclusions from deleted_file_check.php

* Change old J3 comment

* Fix PHPCS unused imports in script.php

Author: richard67