Use new spawn API in keybinding_menu example

[Shatur]

Targets bevy-0.16-dev. I've split this into a separate PR since it's unrelated to the migration and is purely a refactoring.
I like the new API - instead of having multiple setup_* functions, we now have functions that directly return hierarchies.

I recommend to review it after #75.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.03%. Comparing base (2ff798a) to head (91776c3).
Report is 7 commits behind head on bevy-0.16-dev.

Additional details and impacted files

@@              Coverage Diff               @@
##           bevy-0.16-dev      #86   +/-   ##
==============================================
  Coverage          87.03%   87.03%           
==============================================
  Files                 33       33           
  Lines               1003     1003           
==============================================
  Hits                 873      873           
  Misses               130      130           

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