Alerting: Prevent uid collision in migration when db is case-insensitive (#60494)

* Alerting: Prevent short uid collision in legacy migration when db is case-insensitive

Two factors come into play that cause sporadic uid conflicts during legacy alert migration:
- MySQL and MySQL-compatible backends use case-insensitive collation.
- Our short uid generator is not a uniform RNG and generates uids in such a way that generations in quick succession have a higher probability of creating similar uids.

Normally we would be guaranteed unique short uid generation, however if the source alphabet contains
duplicate characters (for example, if we use case-insensitive comparison) this guarantee is void.

Generating even ~1000 uids in quick succession is nearly guaranteed to create a case-insensitive
duplicate.

pkg/services/sqlstore/migrations/ualert/testing.go

@@ -16,6 +16,8 @@ func newTestMigration(t *testing.T) *migration {
 
 			Logger: log.New("test"),
 		},
-		seenChannelUIDs: make(map[string]struct{}),
+		seenUIDs: uidSet{
+			set: make(map[string]struct{}),
+		},
 	}
 }