[MNT] [Dependabot](deps): Update sphinx-gallery requirement

Updates the requirements on [sphinx-gallery](https://github.com/sphinx-gallery/sphinx-gallery) to permit the latest version.
- [Release notes](https://github.com/sphinx-gallery/sphinx-gallery/releases)
- [Changelog](https://github.com/sphinx-gallery/sphinx-gallery/blob/master/.github_changelog_generator)
- [Commits](sphinx-gallery/sphinx-gallery@v0.0.3...v0.18.0)

---
updated-dependencies:
- dependency-name: sphinx-gallery
  dependency-type: direct:production
...

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

pyproject.toml

@@ -72,7 +72,7 @@ docs = [
     "numpydoc",
     "pydata-sphinx-theme",
     "sphinx-issues<5.0.0",
-    "sphinx-gallery<0.18.0",
+    "sphinx-gallery<0.19.0",
     "sphinx-panels",
     "sphinx-design<0.7.0",
     "Sphinx!=7.2.0,<9.0.0",