Change default value for integrated error tracking

What does this MR do and why?

Change the default backend for Error Tracking feature. Because Sentry backend requires you to have Sentry instance (self-hosted or cloud), to use the error tracking feature. So we default to GitLab backend as its easier to start with. Users can always switch to Sentry backend if they want to.

NOTE: user still need to enable error tracking feature with "Active" checkbox

Issue #329596 (closed)

Screenshots or screen recordings

Before Sentry was the default backend, now its GitLab:

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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