deps: update actions/checkout action to v4 (#653)

* deps: update actions/checkout action to v4

* 🦉 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-dependencies.yaml

@@ -17,7 +17,7 @@ jobs:
         - java-storage
         - java-pubsub
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac # v4
     - uses: actions/setup-java@v3
       with:
         distribution: zulu
@@ -30,7 +30,7 @@ jobs:
   flatten-plugin-check:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac # v4
     - uses: actions/setup-java@v3
       with:
         distribution: zulu

.github/workflows/downstream-maven-plugins.yaml

@@ -22,7 +22,7 @@ jobs:
         - lint  # fmt-maven-plugin and google-java-format
         - clirr # clirr-maven-plugin
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac # v4
     - uses: actions/setup-java@v3
       with:
         distribution: zulu