Remove <7 constraint from requirement file

As we're expecting Sphinx to have more major version changes in the near future and this will make things smoother.

Co-authored-by: Jakob Lykke Andersen <jakobandersen@users.noreply.github.com>

requirements/production.txt

@@ -2,4 +2,4 @@ docutils>=0.12
 Jinja2>=2.7.3
 MarkupSafe>=0.23
 Pygments>=1.6
-Sphinx>=4.0,<7,!=5.0.0
+Sphinx>=4.0,!=5.0.0