This PR is [resolving the dispatch part of Coder
Inbocx](https://github.com/coder/internal/issues/403).
Since the DB layer has been merged - we now want to insert notifications
into Coder Inbox in parallel of the other delivery target.
To do so, we push two messages instead of one using the `Enqueue`
method.
The experimental functions in `golang.org/x/exp/slices` are now
available in the standard library since Go 1.21.
Reference: https://go.dev/doc/go1.21#slices
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>
