Remove experimental state from support for namespace in path for pages

Summary

The experiment is currently rolled out to 100% of users and has been deemed a success. The changes need to become an official part of the product.

Related parent issue: GitLab Pages without DNS wildcard - MVC (#17584 - closed)

Steps

• Determine whether the feature should apply to SaaS and/or self-managed: Self-managed and only supports Linux package installation method
• Determine whether the feature should apply to EE - and which tiers - and/or Core: Core (Available for community edition also)
• Determine if tracking should be kept as is, removed, or modified.
• Determine if any UX experiences need to be "polished" i.e. updated to further improve the end user experience. This task should be completed by the designated UX counterpart.
  • (placeholder for UX polish work that needs to be completed for this cleanup issue to be considered completed)
• Ensure any relevant documentation has been updated: Docs MR
• Determine whether there are other concerns that need to be considered before removing the feature flag: N/A
  • These are typically captured in the Experiment Successful Cleanup Concerns section of the rollout issue.
• Consider changes to any feature_category: introduced by the experiment if ownership is changing (PM for Growth and PM for the new category as DRIs): N/A
• Check to see if the experiment introduced new design assets. Add them to the appropriate repos and document them if needed.
• Optional: Migrate experiment to a default enabled feature flag for one milestone and add a changelog. Converting to a feature flag can be skipped at the ICs discretion if risk is deemed low with consideration to both SaaS and (if applicable) self managed: N/A
• In the next milestone, remove the feature flag if applicable
• After the flag removal is deployed, clean up the feature/experiment feature flags by running chatops command in #production channel
• Ensure the corresponding Experiment Rollout issue is updated