Fix: Opening a modal with scrollbar-gutter: stable shifts the page content and shows white (non grey-overlayed) gutters

[ac-mmi]

This pull request addresses #40659 issue.

I have updated the modal SCSS file to include a fix that targets the content shift issue when the scrollbar-gutter: stable both-edges property is used. Specifically:

• I added a @supports rule that checks if the browser supports scrollbar-gutter: stable both-edges.
• When the modal is open (.modal-open class is applied to the body), the additional padding-right adjustment is removed, preventing the content from shifting unexpectedly.

This fix ensures that the modal behavior remains consistent and that no unnecessary padding is applied when scrollbar-gutter: stable both-edges is present.

Closes #40659

[mdo]

Fixed in v6-dev by #42545, which moves the dialog scroll-lock to :root so it co-locates with scrollbar-gutter: stable (no content shift / white gutter) — the same #40659 this addresses. Closing in favor of the v6 work; thanks for the investigation here.