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>