misc: Upgrade {download/upload}-artifact to v4

v3 was causing a 'Node.js 16 actions are deprecated' error.

Note: download-artifact@v4 must be used with upload-artifact@v4 and
vice-versa.

Change-Id: Icb8ab6d27aed4557be95ce31dd89d4655010968e

.github/workflows/ci-tests.yaml

@@ -127,7 +127,7 @@ jobs:
         # stripping the "build" directory. By adding the "anchor.txt" file, we
         # ensure the "build" directory is preserved.
             - run: echo "anchor" > anchor.txt
-            - uses: actions/upload-artifact@v3
+            - uses: actions/upload-artifact@v4
               with:
                   name: ci-tests-${{ github.run_number }}-testlib-quick-all-gem5-builds
                   path: |
@@ -151,7 +151,7 @@ jobs:
 
         # Checkout the repository then download the gem5.opt artifact.
             - uses: actions/checkout@v4
-            - uses: actions/download-artifact@v3
+            - uses: actions/download-artifact@v4
               with:
                   name: ci-tests-${{ github.run_number }}-testlib-quick-all-gem5-builds
 
@@ -177,7 +177,7 @@ jobs:
         # Upload the tests/testing-results directory as an artifact.
             - name: Upload test results
               if: success() || failure()
-              uses: actions/upload-artifact@v3
+              uses: actions/upload-artifact@v4
               with:
                   name: ci-tests-run-${{ github.run_number }}-attempt-${{ github.run_attempt }}-testlib-quick-${{ steps.sanitize-test-dir.outputs.sanatized-test-dir
                       }}-status-${{ steps.run-tests.outcome }}-output