gitlab-ci: Add macos-arm64 jobs for Ninja and Xcode

For now, leave these jobs out during merge requests.

Rename existing macos-* build and test jobs to macos-x86_64-*, and add macos-arm64-* jobs.

Use our Qt macOS 10.13+ package for all macOS jobs since it provides Qt as a universal binary for x86_64 and arm64.

Backport: release

changed milestone to %3.20.0

added os:macos triage:env-regression workflow:in-review labels

Warnings:

• Eligible for the 3.20.x branch.

The warnings do not need to be fixed, but it is recommended to do so.

requested review from @ben.boeckel

In addition to this MR's pipeline (which is not expected to have the new jobs), I ran some pipelines in my fork that simulate the nightly stage pipeline and the nightly package pipeline. The jobs can be seen in the nightly stage pipeline.

+1 otherwise

resolved all threads

Do: merge

removed workflow:in-review label

merged

mentioned in commit 86977011

mentioned in commit 6714074c