Document minimum TLS requirements.

Users might decide to add minimum version requirements on their frontend load balancers, which can break federation to instances that don't said minimums.

Once we have that we should add a check to the federation tester to warn if the TLS requirements are too strict.

Start by locating the Matrix federation specification and the federation tester mentioned in the issue. Determine where minimum TLS requirements should be documented and where the tester can warn about stricter requirements; done means both the documentation and warning behavior are defined and covered by the relevant checks.

Written by the indexing model from the issue text.