Enable restrict_special_characters_in_namespace_path feature flag

What does this MR do and why?

The feature was rolled out on gitlab.com. We are keeping this flag for a bit longer to avoid any surprises and we will remove the feature flag in the %16.5 milestone.

How to set up and validate locally

Follow these steps: !111017 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #390954 (closed)