Write a support entry on Tor Browser dropping support for legacy operating systems

In light of tpo/applications/tor-browser#43125 (closed), let's add some info here for users stuck on legacy operating systems:

Exactly which platforms are affected.
Describe how we're forking Tor Browser users between 13.5-extended and 14.0.
Why we're dropping support for legacy platforms – decisions upstream in Firefox.
Current extent of the extended support window (March 2025), which will be re-evaluated by Mozilla early next year
Why it's important to keep your OS and browser up to date (security vulnerabilities, circumvention features etc.)