Update sphinx-issues requirement from <5 to <6

Updates the requirements on [sphinx-issues](https://github.com/sloria/sphinx-issues) to permit the latest version.
- [Commits](sloria/sphinx-issues@0.1.0...5.0.0)

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

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

pyproject.toml

@@ -43,7 +43,7 @@ dev = [
 ]
 docs = [
     "sphinx<9",
-    "sphinx-issues<5",
+    "sphinx-issues<6",
     "furo==2024.8.6",
 ]