Rework progress bar markup and styles (#36831)

* Rework progress bar markup and styles

Logically moves the various `role` and `aria-` attributes to the `.progress` element itself, leaving the `.progress-bar` to be used purely for the visual presentation. This fixes the problem #36736 that in certain browser/AT combinations, zero-value/zero-width progress bars are completely ignored and not announced.

For multiple/stacked progress bars, this PR introduces a new wrapper and class `.progress-stacked`, to accommodate for the fact that with the more logical structure above, we need full `.progress` elements with child `.progress-bar` elements, and can't get away with the fudge we had before of having a single `.progress` with multiple `.progress-bar`s.

Note that the old markup structures still work with this change, so this could be considered a non-breaking change - though one we definitely want to highlight as it's more accessible (as it now guarantees that zero-value/zero-width progress bars, whether on their own or as part of a multi/stacked bar, are actually announced)

* Add a note about progress bar change in migration guide

* Add notes with old markup examples and explanation

* Fix bundlewatch

* Update site/content/docs/5.2/components/progress.md

Co-authored-by: Julien Déramond <[email protected]>

* Reintroduce deleted styles

Turns out they're needed for correct positioning of text inside progress bar

* Move changes in markup to Migrationg guide, link to that from top of progress page, rewrite some content

* Fix typo in callout

* Clarify "Sizing" section

* Remove redundant "now"

Co-authored-by: Julien Déramond <[email protected]>
Co-authored-by: Julien Déramond <[email protected]>
Co-authored-by: Mark Otto <[email protected]>
Co-authored-by: Mark Otto <[email protected]>

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