feat(GlAlert): drop contained behavior

• Integration MR (does not require this to be merged first): gitlab!81167 (merged).

What does this MR do?

This essentially reverts !1963 (merged) where we had added some logic for GlAlert's content could be constrained to a specific width in fluid layouts.

Before (contained)	After

This follows up on discussions in gitlab!79235 (comment 823111056) where we've decided that this behavior did not meet the design system's expectations.

BREAKING CHANGE: This drops GlAlert's contained behavior.

Alerts content width cannot be constrainted with the contained prop anymore. Existing alerts' content will exapnd to the take up the alert's full width. You should consider removing all contained usages from your code.

Closes #1696 (closed)