* Add dropright (right placement for our dropdown)

* Add dropleft

* moves drop left arrow to the left

* Add .modal-dialog-centered for optional vertically cenetered modal

Fixes #23638

* adds modal-dialog-centered class to docs and removes margin to avoid generating a vertical scrolling

* mention limitations

* fix aria attr

* Add `width: 100%` to the .modal-content for the centered version. Adding it here to avoid adding another selector by limiting it to the centered modal modifier.

Fixes #24503 by manually calling the border-radius instead of using the mixin.

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

The option is set to `always-single-line` in stylelint-config-standard which we extend.

* Fix for right padding in dismissible alerts

* Removed extra whitespace

* Update _alert.scss

* Update _alert.scss

We don't change anything so the transition is basically unused.

* add margin-bottom to .btn of a vertical button group

* match margins of radio/checkbox buttons in a btn-group to the actual buttons

* add comment

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.

As the diff shows, this was for a Firefox Android issue that's since been fixed.

Closes #23817.

Closes #24437, closes #22457

* Update the form focus mixin to use a manual `$enable-shadows` check so we can always ensure a focus state for accessibility and consistency

* - Add new `$input-btn-focus-width` and `$input-btn-focus-color` variables.

- Replace separate `$btn-focus-box-shadow` and `$input-focus-box-shadow`
variables with unified `$input-btn-focus-box-shadow` to match our
combined variables approach elsewhere.

* Put new focus width var to use in buttons mixins

* use new button input-box shadow var

* Add a new mixin for quickly adding linear gradient to components when $enable-gradients is set to true

* use correct var

* fix focus shadows in button mixins

* Add opt-in gradients to alerts, buttons, carousel, custom radios and checkboxes, custom file input, and dropdown items

* Generate .bg-gradient- utilities

* add headings to colors page and document bg-gradient utils

* update the button color for active status, check with yiq as it's done for basic state and hover state

This way, descenders are not cropped when truncating. Fixes #23956.

* font-sizes relative to font-size-base

* ditch $heading-font-size-base, simplify

* apply same to .lead var

* also update progress bar font-size var

* fixes migration forms guides

* removes container from Horizontal form

* adds col-form-label sizes on form docs

* removes .form-control-label from forms.scss

* add note to migration