Remove the generic_packages feature flag

🃏 Context

The generic_packages feature flag has been used as a safety net during the deployment of the Generic Package Registry.

It has been enabled by default for more than a year.

I think it's prime time to remove it from the codebase.

🔍 What does this MR do and why?

• Remove the generic_packages feature flag.
• Update the related spec.
• Update the related documentation.

🖥 Screenshots or screen recordings

n / a

⚙ How to set up and validate locally

You can try to use the generic registry as documented in https://docs.gitlab.com/ee/user/packages/generic_packages/.

🚥 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.