Closes #24341

Closes #25067

Explicitly re-adds the "hand" `cursor:pointer` for non-disabled `.btn`, `.close`, `.navbar-toggler` elements, as well as forcing `page-link` pagination controls to always have the "hand" cursor (even if an author uses it on, say, `<button>` elements for a dynamically updating in-page pagination).

Controversial, as everybody jumped on the bandwagon following this article https://medium.com/simple-human/buttons-shouldnt-have-a-hand-cursor-b11e99ca374b - which does have its merits of course, but there are also counter-arguments like http://kizu.ru/en/issues/cursor-pointer/

And seeing the amount of issues we've seen following the change, and the potential complexity needed to consistently address the cursor issue (see https://github.com/twbs/bootstrap/issues/24156 where i explore how the cursor needs to be based on "intent", and how that's tough to determine), I'm favouring an opinionated take here of just reintroducing the `cursor:pointer`.

- Changes the wrapping label to a div so we can style the label instead of another element while also supporting form validation.
- Fixes form validation styles for custom file input (closes #24831).
- Updates docs with validation styles (also adding example feedback text while I was there) and new how it works section.

* Remove unnecessary float

- since parent btn-group is displayed as inline-flex, the child elements are placed left to right by default.

* Cleanup of reset rounded corners

- Remove `border-radius :0` since it is enough by each reset of left and right side
- A `.dropdown-toggle` selector should not be used for reset rounded corner since it does not have border-radius property. Instead use .dropdown-toggle as negation pseudo-class.

* Fix input group border radius issue (#25075)

* Remove migration note

- All buttons In input group do not have to be the same element now.

* lint

Fixes #25016 which noticed a .thead-light and border-color of the .table-border are the same color.