Forms: Avoid the non-clickable gutter between `form-check-input` and its label

[ThomasLandauer]

The area between a radiobutton and its label is not clickable:

(Screenshot taken from https://getbootstrap.com/docs/5.3/forms/checks-radios/)

This is due to the margin of form-check-input (shown in yellow):

[mdo]

We must've had dozens of issues around this over the years, so maybe it's time to just do it. I'll tag it for v6 for now.