snackbar-dialog: color and layout updates

[randybascue]

We've updated the snackbar styling to increase contrast against the background. We've also updated the placement guidelines on larger screens.

Figma page Playbook page

[ianmcburnie]

Dev note: Possible markup change required here to accommodate positioning of that info icon, which could mean a small breaking change. Therefore v14 release might make sense.

[agliga]

Should do next version. Need to double check if breaking but does not seem like it will be breaking.