Merge branch '6373-el9x-support' into 'develop'

Resolve "Add support for RHEL 9 and compatible clones"

Closes #6373

See merge request !1656
1 job for 6393-websites-php-version-gets-accidentally-altered-by-admin-if-websites-php-version-is-client_id-0 in 9 seconds (queued for 1 second)