gitlab-ci: upgrade macOS runners

gitlab-ci: upgrade macOS runners

We're currently using M1-based runners for our macOS jobs. GitLab has
since introduced a new M2 Pro-based runner type that is available for
all GitLab tiers [1], which upgrades from 4 to 6 cores and from 8 to 16
GB RAM.

Upgrade to this new runner type, which results in some nice speedups:

  - osx-clang goes from 26 minutes to 16 minutes.

  - osx-meson goes from 19 minutes to 13 minutes.

  - osx-reftable goes from 23 minutes to 14 mintues.

[1]: https://docs.gitlab.com/ci/runners/hosted_runners/macos/

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>