README.rst: sync eol confluence versions

Confluence versions before 7.16 have reached end-of-life and will not be
tested in next release cycle system tests; updating to the oldest
version supported.

Signed-off-by: James Knight <[email protected]>

Author: jdknight

README.rst

@@ -26,7 +26,7 @@ optionally publish them to a Confluence instance.
 Requirements
 ============
 
-* Confluence_ Cloud or Data Center / Server 7.14+
+* Confluence_ Cloud or Data Center / Server 7.16+
 * Python_ 3.8+
 * Requests_ 2.14.0+
 * Sphinx_ 6.1+