deps: update actions/checkout action to v3 (#1593)

* deps: update actions/checkout action to v3

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

.github/workflows/downstream.yaml

@@ -133,7 +133,7 @@ jobs:
         - workflow-executions
         - workflows
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - uses: actions/setup-java@v1
       with:
         java-version: ${{matrix.java}}