Validate the parent of a personal namespace/Group by default

What does this MR do and why?

The feature flag validate_namespace_parent_type was introduced in !54094 (merged).

With the rollout in #322101 (closed), it is now fully enabled on gitlab.com and now we want to roll this out to self-managed installations as well.

Current status:

However, there is a probability that some self-managed instances may encounter trouble when this validation goes live, so we should also provide the flexibility to such instances to turn this flag off if required.

This MR adds default_enabled: true to the feature flag check, so that self-managed instances can turn off the flag if they need to.

We have also updated the docs to include the steps on how to turn off this flag.

Screenshots or screen recordings

How to set up and validate locally

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