Require HTTP2 for major browsers

[unixfox]

A good protection is to require HTTP2 for the major browsers (based on the user-agent).

Mainly because those major browsers are now only sending requests over HTTP2 or HTTP3: https://caniuse.com/http2 and https://superuser.com/a/1659262

Requiring HTTP2 for all requests could work too, but has to be thought before implementing that because I don't know about the potential side effects.