Update Rapid Diffs feedback issue URL constant

What does this MR do and why?

For #596239 (closed)

Updates the URL for Rapid Diffs feedback to be the correct issue.

How to set up and validate locally

I mean, you can probably just review the code in isolation, because... You know, just look at it.

But alright, here are validation steps:

1. Enable the feature flag:

   Feature.enable(:rapid_diffs_on_mr_show)

2. Click the Try Rapid Diffs button - the page should reload with Rapid Diffs active
3. The button is now a Rapid Diffs dropdown with three options
4. Click Leave feedback

MR acceptance checklist

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