Update upload-artifact action to version 4 in buildtest workflow

Author: tg123

.github/workflows/buildtest.yaml

@@ -31,7 +31,7 @@ jobs:
           directory: ./TestResults
           files: '*.cobertura.xml'
       - name: Upload test results
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: test-results
           path: ./TestResults