ci: fix artifact action versions (v6 -> v4)

Author: zales

.github/workflows/main.yml

@@ -110,7 +110,7 @@ jobs:
           generate_release_notes: true
 
       - name: Upload Artifacts
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v4
         with:
           name: firmware
           path: web_flasher/
@@ -124,7 +124,7 @@ jobs:
     if: startsWith(github.ref, 'refs/tags/')
     steps:
       - name: Download Artifacts
-        uses: actions/download-artifact@v6
+        uses: actions/download-artifact@v4
         with:
           name: firmware
           path: .