Support Go 1.21

[stefanb]

Homebrew/homebrew-core#134468 found a problem with Go 1.21
https://github.com/Homebrew/homebrew-core/actions/runs/5618619997/job/15224884480?pr=134468#step:3:3203
reported error:

# github.com/quic-go/quic-go/internal/qtls
/Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/quic-go/[email protected]/internal/qtls/go121.go:5:13: cannot use "The version of quic-go you're using can't be built on Go 1.21 yet. For more details, please see https://github.com/quic-go/quic-go/wiki/quic-go-and-Go-versions." (untyped string constant "The version of quic-go you're using can't be built on Go 1.21 yet. F...) as int value in variable declaration

Go 1.21 support has been added in quic-go 0.37.0: https://github.com/quic-go/quic-go/releases/tag/v0.37.0 but it requires few code changes in brook.

[stefanb]

https://github.com/quic-go/quic-go/releases/tag/v0.37.1 fixed a few regressions

[stefanb]

https://github.com/quic-go/quic-go/releases/tag/v0.37.3 has few more fixes