Removes milestone from cluster base domain documentation

What does this MR do?

12.0 milestone was removed from cluster base domain on AutoDevOps documentation so it's aligned with the comments on the documentation.

Related issues

From https://gitlab.com/gitlab-org/gitlab-ce/issues/57397#note_139404596

Author's checklist

• Apply the correct labels and milestone
• [-] Crosslink the document from the higher-level index
• [-] Crosslink the document from other subject-related docs
• [-] Feature moving tiers? Make sure the change is also reflected in features.yml
• [-] Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards