Update tested versions in README

Update the Wagtail, Django, and Python versions mentioned as being
tested in the README based on the tox configuration.

This section previously used the greater-than-or-equal sign (`>=`) when
the less-than-or-equal sign (`<=`) was intended. Because these symbols
are potentially confusing, they have been removed.

Author: martey

README.rst

@@ -31,9 +31,9 @@ wagtailmenus is an extension for Torchbox's `Wagtail CMS <https://github.com/tor
 
 The current version is tested for compatibility with the following:
 
-- Wagtail versions >= 5.2
-- Django versions 4.2, 5.0 and >= 5.1
-- Python versions 3.9 to 3.12
+- Wagtail versions from 5.2 to 6.3
+- Django versions 4.2, 5.0 and 5.1
+- Python versions from 3.9 to 3.13
 
 .. image:: https://raw.githubusercontent.com/jazzband/wagtailmenus/master/docs/source/_static/images/repeating-item.png