Disallow integrated error tracking by default

What does this MR do and why?

This introduces a FF to hide the Integrated Error Tracking functionality. It is disabled by default (integrated error tracking is not available). This is part of #353639 (closed).

The flag does a few things when disabled:

• Hides the UI to add an integrated GitLab error tracking token
• Returns a 404 error in the Error Tracking Collector APIs used by the integrated error tracking
• Returns "integrated": false in the Error Tracking settings API.

Screenshots or screen recordings

FF disabled (w/ banner for users who had enabled integrated)	FF enabled

Error tracking list, FF disabled (w/ banner for users who had enabled integrated)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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