OS / browser testing to determine minimum viable HTTP version

[devinrsmith]

Spawned from #2731, which was too aggressive in trying to disable HTTP1.1.

If we want to disable HTTP1.1 by default in some cases, we should make sure the web UI continues to work across common OS and browser versions.

We may be able to use something like https://www.ssllabs.com/ssltest/, or similar services, to test a wide range of OS / browsers / versions.

[devinrsmith]

From Charles:

what about non-browser clients?

From Don:

https://caniuse.com/http2 looks broadly supported across all modern browsers, and appears to meet the browserlists default "> 0.5%, last 2 versions, Firefox ESR, not dead" which we use as the determining factor for when we can adopt new browser features.