Add UniverseDomain configuration for GCS cache

What does this MR do?

Document the UniverseDomain parameter added in !6338 (merged) for Google Cloud Storage cache configuration. This parameter enables support for Google Cloud Dedicated and other custom universe domains.

What's the best way to test this MR?

You'll need a storage bucket and VM on a Google Dedicated Cloud account. For example:

  [runners.cache]
    MaxUploadedArchiveSize = 0
    type = "gcs"
    [runners.cache.s3]
    [runners.cache.gcs]
    BucketName = 'example-bucket'
    UniverseDomain = 'apis-berlin-build0.goog'

Then run a CI job that uploads and downloads from the cache:

test:
  image: busybox:latest
  script:
    - echo "hello world" > world.txt
  cache:
    paths:
      - world.txt

What are the relevant issue numbers?

Relates to https://gitlab.com/gitlab-org/gitlab/-/work_items/581881