Bump panel from 1.3.8 to 1.4.2 in /requirements

Bumps [panel](https://github.com/holoviz/panel) from 1.3.8 to 1.4.2.
- [Release notes](https://github.com/holoviz/panel/releases)
- [Changelog](https://github.com/holoviz/panel/blob/main/CHANGELOG.md)
- [Commits](holoviz/panel@v1.3.8...v1.4.2)

---
updated-dependencies:
- dependency-name: panel
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

requirements/requirements_doc.txt

@@ -20,5 +20,5 @@ sphinx-gallery==0.15.0
 sphinx-autobuild==2024.2.4
 sphinx-autodoc-typehints==2.0.0
 sphinx-notfound-page==1.0.0
-panel==1.3.8
+panel==1.4.2
 pypandoc==1.13