http3 upstream connection stalls

When using an HTTP3 upstream (tested with `h3://dns.google/dns-query`) sometimes the dns proxy stops responding to queries.

The proxy starts working again after `dns.google` closes the connection due to max connection age being reached. At that point the proxy restarts a new connection to `dns.google` using 0-RTT.

This is happening just using this proxy as the DNS for my local machine. So far I have not been able to create a scenario for reliably reproducing the issue, but it'll usually start occurring after some time (maybe at least 20 minutes).

From the verbose logs I can see that the proxy gets stuck at:
https://github.com/AdguardTeam/dnsproxy/blob/f661fdcf9edc0ddfebf6465b90d2b77314533980/upstream/doh.go#L223