Adjust how we do version text in docs

What does this MR do?

Provides certainty to documentation authors about when it's appropriate to add and remove text that specifically mentions GitLab versions.

Author's checklist

• Apply the correct labels and milestone
• 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