Release "allow anyone to pull" feature for terraform modules

What does this MR do and why?

In Make "allow anyone to pull" work with group-lev... (!166334 - merged), we introduced the ability to pull modules from a public Terraform Registry regardless of its group and project visibility. That means we can have a private project, but when enabling its Allow anyone to pull setting, anyone on the internet can download modules from this public registry.

The feature was gated behind a feature flag, and it was globally enabled on gitlab.com without any issues.

In this MR, we clean up the feature flag, and update the documentation to release the feature.

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Related to #468062 (closed)