Bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/build_offline_docs.yml

@@ -58,7 +58,7 @@ jobs:
       - name: Sphinx - Build HTML
         run: make SPHINXOPTS='--color -j 4' html
 
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         with:
           name: godot-docs-html-${{ matrix.branch }}
           path: _build/html
@@ -80,7 +80,7 @@ jobs:
 
           make SPHINXOPTS='--color -j 4' epub
 
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         with:
           name: godot-docs-epub-${{ matrix.branch }}
           path: _build/epub/GodotEngine.epub