chore(deps): update actions/download-artifact action to v3 (#77)

Source-Link: googleapis/synthtool@38e11ad
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:4e1991042fe54b991db9ca17c8fb386e61b22fe4d1472a568bf0fcac85dcf5d3

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,4 +13,4 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-python:latest
-  digest: sha256:5d8da01438ece4021d135433f2cf3227aa39ef0eaccc941d62aa35e6902832ae
+  digest: sha256:4e1991042fe54b991db9ca17c8fb386e61b22fe4d1472a568bf0fcac85dcf5d3

.github/workflows/unittest.yml

@@ -26,7 +26,7 @@ jobs:
       run: |
         nox -s unit-${{ matrix.python }}
     - name: Upload coverage results
-      uses: actions/upload-artifact@v2
+      uses: actions/upload-artifact@v3
       with:
         name: coverage-artifacts
         path: .coverage-${{ matrix.python }}
@@ -47,7 +47,7 @@ jobs:
         python -m pip install --upgrade setuptools pip wheel
         python -m pip install coverage
     - name: Download coverage results
-      uses: actions/download-artifact@v2
+      uses: actions/download-artifact@v3
       with:
         name: coverage-artifacts
         path: .coverage-results/