util/eventbus: make internal queue a generic type

In preparation for making the queues carry additional event metadata.

Updates #15160

Signed-off-by: David Anderson <dave@tailscale.com>

util/eventbus/bus.go

@@ -78,7 +78,7 @@ func (b *Bus) Close() {
 }
 
 func (b *Bus) pump(ctx context.Context) {
-	var vals queue
+	var vals queue[any]
 	acceptCh := func() chan any {
 		if vals.Full() {
 			return nil