Default enable FF: self_managed_code_suggestions_completion_api

What does this MR do and why?

Enable self_managed_code_suggestions_completion_api by default.

This FF guards the /completions endpoint for Self Managed.

No-op for SaaS, only SM instances are affected. The flag was only introduced during 16.3 dev.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/420599, #418795 (closed)

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Related to #418795 (closed)