test(deps): update actions/checkout action to v4 (googleapis#1899)

* 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/conformance.yaml

@@ -23,8 +23,8 @@ jobs:
   conformance:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
+    - uses: actions/checkout@v4
       with:
         repository: googleapis/cloud-bigtable-clients-test
         ref: main

.github/workflows/unmanaged_dependency_check.yaml

@@ -5,7 +5,7 @@ jobs:
   unmanaged_dependency_check:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: actions/setup-java@v3
       with:
         distribution: temurin