Fix: code suggestions in admin settings

What does this MR do and why?

Fix: code suggestions in admin settings

• Add an empty template to override the suggestion tab in admin settings.

Add a new FF to control the display of instance_level_code_suggestions_enabled admin setting.

FF is turned off by default and does not display the setting form.

Upstream MR of adding instance_level_code_suggestions_enabled setting: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/121565

JH Rollout FF issue: #3661

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