Support `h2c` (HTTP/2) for listeners, to mitigate against HTTP desync attacks

When a listener's type is `http` and tls is `false`, please also support `h2c` protocol. Currently, only HTTP 1.1 is supported in this case. When trying to connect via HTTP/2, there are errors in the log and the request is ignored.

Client:
```
# docker compose exec synapse curl --http2-prior-knowledge http://localhost:8008/
curl: (16) Remote peer returned unexpected data while we expected SETTINGS frame. Perhaps, peer does not support HTTP/2 properly.
```

Server log:
```
synapse.http.site - 422 - INFO - sentinel - Connection from IPv6Address(type='TCP', host='::1', port=50648, flowInfo=0, scopeID=0) lost before request headers were read
```

HTTP/2 is useful for running behind reverse proxies when they commonly terminate the SSL, so the connection from reverse proxy to Synapse is not transport encrypted.