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)

scss/_progress.scss

@@ -8,7 +8,8 @@
 }
 // scss-docs-end progress-keyframes
 
-.progress {
+.progress,
+.progress-stacked {
   // scss-docs-start progress-css-vars
   --#{$prefix}progress-height: #{$progress-height};
   @include rfs($progress-font-size, --#{$prefix}progress-font-size);
@@ -30,9 +31,6 @@
 }
 
 .progress-bar {
-  display: flex;
-  flex-direction: column;
-  justify-content: center;
   overflow: hidden;
   color: var(--#{$prefix}progress-bar-color);
   text-align: center;
@@ -46,6 +44,14 @@
   background-size: var(--#{$prefix}progress-height) var(--#{$prefix}progress-height);
 }
 
+.progress-stacked > .progress {
+  overflow: visible;
+}
+
+.progress-stacked > .progress > .progress-bar {
+  width: 100%;
+}
+
 @if $enable-transitions {
   .progress-bar-animated {
     animation: $progress-bar-animation-timing progress-bar-stripes;