README.md: bump sphinx version (v7.0+)

Most recent work only aims to support v7.0+ series.

See CONTRIBUTING.rst for additional information.

Signed-off-by: James Knight <james.d.knight@live.com>

README.md

@@ -13,7 +13,7 @@ files and optionally publish them to a Confluence instance.
 * [Confluence][confluence] Cloud or Data Center 7.19+
 * [Python][python] 3.8+
 * [Requests][requests] 2.25.0+
-* [Sphinx][sphinx] 6.1+
+* [Sphinx][sphinx] 7.0+
 
 ## Installing