Add count of older messages in Snackbar alert

Adds "(n older messages)" suffix to the message contents in the
core AlertDisplay when there are multiple messages to show. Some
considerations that led to this:

* It seems like if we're going to show only one message, it should be
  the most recent message (which is the current behavior).
* Just showing a message count (e.g. '1/4', '4/4') seems open
  to interpretation about whether the count starts at the oldest or
  newest.

Signed-off-by: Mike Lewis <mtlewis@users.noreply.github.com>

.changeset/stale-gifts-push.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core': patch
+---
+
+Add count of older messages when multiple messages exist in AlertDisplay