Closes https://github.com/twbs/bootstrap/issues/25316

- Drops the hover-focus mixin for standard :hover
- Adds explicit :focus styles to match button, input, etc with a box-shadow
- Adjusts z-index values to ensure the layers go initial, hover, active, focus

Fixes #24838.

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`.

1. move a couple of them before the more specific ones.
2. change nesting to be under the same parent selector
3. use the ampersand more

Reduce z-indexes in button-group, input-group, list-group, and pagination to the minimum necessary (#24315)

These were using `z-index: 2` to "Place active items above their
siblings for proper border styling". However, using `z-index: 1` is
sufficient for accomplishing that goal.

In input-group, there were also three `z-index: 3` rules for the
hover/focus/active states. I reduced these to `z-index: 2` since they
just needed to be "one more than normal" (i.e. one more than what is now
`z-index: 1` after my changes).

These changes can be verified by viewing the documentation pages for
Button group, Input group, List group, and Pagination before and after
this commit and observing that the active elements are still "above"
their siblings, so their borders look correct.

* Rename variable $input-border-focus to $input-border-color-focus
* Rename variables $pagination-hover-border, $pagination-active-border, $pagination-disabled-border to $pagination-hover-border-color, $pagination-active-border-color, $pagination-disabled-border-color, respectively
* Rename variables $state-*-border to $state-*-border-color, respectively
* Rename variables $alert-*-border to $alert-*-border-color
* Rename $list-group-active-border to $list-group-active-border-color
* Rename $table-inverse-border to $table-inverse-border-color
* Rename $btn-*-border to $btn-*-border-color
* Rename $navbar-*-toggler-border to $navbar-*-toggler-border-color

this, coupled with the move to flex, fixes #20029

Fixes #21094: Move text-decoration override from the default link state to the hover/focus state so it can override the global styles properly

- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

as currently this was missed out (compared to disabled `a.btn`)

[skip sauce]

- no real need for everything to be 100% shared
- padding looked and felt too large for inputs but not for buttons
- tying forms and buttons seems fine, but throwing in pagination feels wrong

We have it set to `1` right now just to avoid compilation errors, but
it’s been replaced everywhere with our new spacer classes anywho. We’ll
likely want to remap that var to custom component vars though (e.g.,
`$pagination-margin` instead of `$spacer-y`).