Update dismiss_approvals.yml (#1623)

In order to add this check to presubmit, it has to be triggered not only
when a PR is updated, but also when a PR is opened or re-opened, which
are the default types.

Author: meteorcloudy

.github/workflows/dismiss_approvals.yml

@@ -1,7 +1,7 @@
 name: Dismiss Stale PR Approvals
 on:
   pull_request_target:
-    types: [synchronize]
+    types: [opened, synchronize, reopened]
     branches:
       - main
     paths: