[modal] Support scrollbar-gutter about material-ui

[oliviertassinari]

@DiegoAndai I removed the Material UI v7 milestone: There is no need for a breaking change, this could be fixed tomorrow, and it's Base UI that needs to be fixed.

Also changed the [tag] to match with the GitHub label ([modal]).

[oliviertassinari]

A side note, a similar outcome can be achieved with the overflow-y style:

html {
  overflow-y: scroll;
}

but it's not ideal, see the difference in UX (Windows):

Screen.Recording.2024-01-24.at.00.34.21.mov