Group-level pipeline semaphores (follow up to project-level semaphores for pipelines)

When #20481 (moved) is implemented, it will be possible to limit pipeline execution based on semaphores at the project-level. This is a great feature, but given our extent of support for environments, pipeline variables, Kubernetes clusters, etc at the group level, after we implement semaphores at the project level, we should expand it to be able to be used at the group level.

/cc @jlenny @inorton