Add control over minimum versions of clients

**Description:**

Seeing this CVE on the android client:
https://element.io/blog/security-release-element-android-1-6-12/

We are wondering if it is possible to configure synapse server to only accept android client past this date, and show the client a nice reminder to update to connect again.

(i don't think it is currently possible, hence the feautre request)

Thanks!