ci: pin upload-artifact to v3 for Gitea compatibility

Gitea's act_runner rejects @actions/artifact v2 (the engine behind
upload-artifact@v4). v3 is the last GHES-compatible major and still
supports the path: glob + retention-days we need.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -50,7 +50,7 @@ jobs:
           go tool cover -html=coverage.out -o coverage.html
 
       - name: Upload coverage artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         if: always()
         with:
           name: coverage