Update sphinx-issues requirement from <6 to <7

Updates the requirements on [sphinx-issues](https://github.com/sloria/sphinx-issues) to permit the latest version.
- [Commits](sloria/sphinx-issues@0.1.0...6.0.0)

---
updated-dependencies:
- dependency-name: sphinx-issues
  dependency-version: 6.0.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

pyproject.toml

@@ -13,7 +13,7 @@ dev = [
 ]
 docs = [
     "sphinx<9",
-    "sphinx-issues<6",
+    "sphinx-issues<7",
     "furo==2025.12.19",
 ]
 check-package-metadata = [