ci: replace x86_64 macos-11 with aarch64 macos-12

ci: replace x86_64 macos-11 with aarch64 macos-12

The Cirrus CI service has announced the intent to discontinue
support for x86_64 macOS CI runners. They already have aarch64
runners available and require all projects to switch to these
images before Jan 1st 2023. The different architecture is
merely determined by the image name requested.

For aarch64 they only support macOS 12 onwards. At the same
time our support policy only guarantees the most recent 2
major versions, so macOS 12 is already technically our min
version.

https://cirrus-ci.org/blog/2022/11/08/sunsetting-intel-macos-instances/

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20221116175023.80627-1-berrange@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>