[MNT] [Dependabot](deps): Update sphinx-issues requirement from <5.0.…

…0 to <6.0.0 (#484)

Updates the requirements on
[sphinx-issues](https://github.com/sloria/sphinx-issues) to permit the
latest version.

pyproject.toml

@@ -85,7 +85,7 @@ docs = [
     "pydata-sphinx-theme",
     "sphinx!=7.2.0,<9.0.0",
     "sphinx-design<0.7.0",
-    "sphinx-issues<5.0.0",
+    "sphinx-issues<6.0.0",
     "sphinx-gallery<0.19.0",
     "sphinx-panels",
     "tabulate",