Document minimum TLS requirements.

[erikjohnston]

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.

[clokep]

It seems to me like this should really be a spec thing, not a Synapse thing?

[anoadragon453]

For some context major browsers are dropping support for TLS 1.0 and 1.1 this year[0][1][2][3].

[0] https://webkit.org/blog/8462/deprecation-of-legacy-tls-1-0-and-1-1-versions/
[1] https://security.googleblog.com/2018/10/modernizing-transport-security.html
[2] https://blog.mozilla.org/security/2018/10/15/removing-old-versions-of-tls/
[3] https://blogs.windows.com/msedgedev/2018/10/15/modernizing-tls-edge-ie11/#OkGpBsuyj6XwhUEQ.97

[ShadowJonathan]

Some more context; Conduit prefers compilation with rustle, which heavily prefers v1.3 over v1.2, though it still supports the latter.

A further comment; with the new global spec version release process, I think a sliding window of required TLS versions is possible per those versions, this way, matrix will stay "up to date" with newer TLS developments as they'd happen.