Update README.md - Change SAML status to Unmaintained

[asaf400]

Overview

I would consider 'Unmaintained' as an attribute of the status, so it is clear that users should not use Dex with SAML for new configurations, and migrate existing ones.

Unmaintained is currently in the note, which is prominently visible, but a note is a note, and maintained is a state-us.. :)

A decline of this PR is perfectly acceptable as well 👍