Use upload-artifact@v3 for Gitea compatibility

upload-artifact@v4 requires the GitHub-only artifact v2 API
(GHESNotSupportedError on Gitea). v3 uses the v1 protocol that
Gitea's act_runner implements, and is mirrored on the Forgejo
mirror.

.gitea/workflows/build-pack.yaml

@@ -48,7 +48,7 @@ jobs:
         run: python3 ../tools/validate_pack.py . cf ../SciCraft-CurseForge.zip
 
       - name: Upload pack artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: scicraft-pack
           path: |