ci: upgrade artifact actions to v4

- upload-artifact@v3 -> v4 (removed deprecated retention-days)
- download-artifact@v3 -> v4 with merge-multiple: true
- This flattens artifacts into the download path, simplifying the
  find/upload loop that uploads assets to Gitea

.gitea/workflows/release.yaml

@@ -80,14 +80,11 @@ jobs:
           echo "archive=$archive" >> "$GITHUB_OUTPUT"
 
       - name: Upload build artifact
-        # Gitea Actions doesn't support upload-artifact v4 (uses a
-        # GitHub-only backend API); pin to v3.
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: nuwiki-ls-${{ matrix.target }}
           path: ${{ steps.package.outputs.archive }}
           if-no-files-found: error
-          retention-days: 7
 
   release:
     name: gitea release
@@ -95,11 +92,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Download all build artifacts
-        # Paired with upload-artifact@v3 above. v3 has no merge-multiple
-        # option and nests each artifact under its own subdirectory.
-        uses: actions/download-artifact@v3
+        # upload-artifact@v4 + download-artifact@v4 with merge-multiple
+        # flattens all artifacts into the download path directly.
+        uses: actions/download-artifact@v4
         with:
           path: ./artifacts
+          merge-multiple: true
 
       - name: Ensure jq + curl
         run: |