gitlab-ci: update macOS image

gitlab-ci: update macOS image

The GitLab CI jobs for macOS are all using the macOS 15 images. While
these images are not deprecated yet, there is a new image for macOS 26
generally available by now [1].

Switch two of our jobs to use the new image. The third job still
continues to use the old image. This ensures broader test coverage until
this old image gets deprecated.

[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>