When the AWS Account ID is provided, we must check that it is a valid
one.
AWS Account ID is a well documented field: 12-digit string
This check is meant to prevent typos when creating Databases mainly from
the Web UI in the context of Teleport Discover.
We were inconsistent throughout the codebase and would sometimes
use the slices package and other times use our own equivalents
in api/.
This removes our versions in favor of the golang.org/x package that
does the same, which has the added benefit of reducing the surface
area of the public API module.
Note: despite existing uses of the slices package, for some reason
it didn't show up in go.mod or go.sum. Fixed that too.