Replace file browser shortcut with mod+b

What does this MR do and why?

Implements Replace File browser `f` shortcut (#542204 - closed).

This MR changes file browser shortcut in merge requests from f to mod+b.

The old shortcut will continue to work for some time. If a user presses the old shortcut they would get a message suggesting a switch to the new one.

The shortcut has to be changed because it conflicts with a global f shortcut for focusing filter bar, which we plan to use at some point.

Screenshots or screen recordings

Screen_Recording_2025-07-04_at_00.20.22

How to set up and validate locally

1. Go to any merge request
2. Select 'Changes' tab
3. Press f
4. Observe a toast message saying to switch to a new shortcut
5. Close the toast message
6. Press f again
7. Observe no message