Enable Rapid Diffs on the merge request creation form

What does this MR do and why?

This MR:

1. Changes rapid_diffs and rapid_diffs_on_mr_creation feature flag types from wip to beta
2. Enables Rapid Diffs on the merge request creation form by default

These feature flags will be removed in the next milestone(s).

Screenshots or screen recordings

https://youtu.be/F1WyDYQCARs

How to set up and validate locally

1. Go to merge request list
2. Click 'New merge request'
3. Select branches, proceed onto next page
4. Select 'Changes'
5. Observe Rapid Diffs on the page if feature flags are not overridden by your local config