Drop macOS 11 out from dev (6.8) branch as it is EOL or very near it. This removes macOS 11 test configurations and bumps macOS targets to
12 in provision scripts.
Task-number: QTQAINFRA-6009
Change-Id: I2ea2764fb4a14eb508e39330acfc0947b94343fc
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>