Update upload-artifact action version in diagnostics.yml

Changed the `actions/upload-artifact` GitHub Action from version `v2` to `v4` to ensure compatibility with the latest features and improvements. This update helps maintain the workflow's reliability and performance.

.github/workflows/diagnostics.yml

@@ -69,7 +69,7 @@ jobs:
         run: ./vendor/bin/phpcq run -v -o github-action -o default --threshold=info
 
       - name: Upload build directory to artifact
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v4
         if: ${{ success() }} || ${{ failure() }}
         with:
           name: phpcq-builds-php-${{ matrix.php }}