Fixes #25235

Unintentionally removed in #22703. Fixes #24643.

* Add box-shadow to .progress-bar

Puts an unused variable back in use for when folks set `$enable-shadows` to `true`.

- Pulls in values from v3
- Closes #22371 which didn't use a transition include

* remove obvi comments

* proper fix for #21801

- ensures .progress-bar text is vertically centered should the height of the .progress-bar change by matching the line-height and height values
- we can't use flexbox here because the inner text of an element doesn't count as a flex item

- <progress> element didn't allow animation, labels overlaid, multiple bars, etc.

- Revamps CSS to use something more similar to v3's implementation

- Ditches variant mixin for `bg-` utils

- Rebuilds docs to match, including adding a new Height section for customizing that.

Only potential remaining todo is adding `.sr-only` instances to within the bar. Unsure if that's necessary.

Remove most border-radius properties and instead use overflow: hidden; to properly round corners of a full progress bar. (#21216)

Fixes #19065.

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Erratum from #18658
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fixes #18658: no more min-width action on progress bars. we had them in v3 and ended up removing them for additional problems it caused folks.

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Refs #18535
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Refs #18535

as it's currently not displayed, and (in my opinion) would look fugly if
it did appear...

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Fix #17288 — Sorry about that!

Fix #17052

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`).

Autoprefixer already takes care of this for us.