Globally enable the `:schema_linting` flag

What does this MR do and why?

Globally enables the :schema_linting feature flag. The flag has been enabled globally on gitlab.com for 2 years and this MR is bringing the functionality to the self-managed instances by default.

Note: this MR does not do the cleanup or remove the flag just yet. It will be done in a separate MR.

Screenshots or screen recordings

No visual changes on gitlab.com should be available.

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 #255919 (closed)