Bump sphinx-gallery from 0.20.0 to 0.21.0

Bumps [sphinx-gallery](https://github.com/sphinx-gallery/sphinx-gallery) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/sphinx-gallery/sphinx-gallery/releases)
- [Changelog](https://github.com/sphinx-gallery/sphinx-gallery/blob/master/CHANGES.rst)
- [Commits](sphinx-gallery/sphinx-gallery@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: sphinx-gallery
  dependency-version: 0.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

pyproject.toml

@@ -89,7 +89,7 @@ documentation = [
     "numpydoc==1.10.0",
     "sphinx>=5.1.1",
     "sphinx-rtd-theme==3.1.0",
-    "sphinx-gallery==0.20.0",
+    "sphinx-gallery==0.21.0",
     "sphinx_design==0.6.1",
     "pydata-sphinx-theme==0.16.1"
 ]