build(deps): bump github.com/quic-go/quic-go from 0.40.1 to 0.42.0 (#289)

* build(deps): bump github.com/quic-go/quic-go from 0.40.1 to 0.42.0

Bumps [github.com/quic-go/quic-go](https://github.com/quic-go/quic-go) from 0.40.1 to 0.42.0.
- [Release notes](https://github.com/quic-go/quic-go/releases)
- [Changelog](https://github.com/quic-go/quic-go/blob/master/Changelog.md)
- [Commits](https://github.com/quic-go/quic-go/compare/v0.40.1...v0.42.0)

---
updated-dependencies:
- dependency-name: github.com/quic-go/quic-go
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

* update: remove dependency of quic-go

We now vendor the quicvarint submodule from quic-go for a minimal dependency tree.

This also updates the minimal Go version requirement to Go 1.21, given uTLS promised to support 2 most recent minor versions of Go.

Signed-off-by: Gaukas Wang <i@gaukas.wang>

---------

Signed-off-by: dependabot[bot] <support@github.com>
Signed-off-by: Gaukas Wang <i@gaukas.wang>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Gaukas Wang <i@gaukas.wang>

go.mod

@@ -1,6 +1,6 @@
 module github.com/refraction-networking/utls
 
-go 1.20
+go 1.21
 
 retract (
 	v1.4.1 // #218
@@ -11,7 +11,6 @@ require (
 	github.com/andybalholm/brotli v1.0.6
 	github.com/cloudflare/circl v1.3.7
 	github.com/klauspost/compress v1.17.4
-	github.com/quic-go/quic-go v0.40.1
 	golang.org/x/crypto v0.18.0
 	golang.org/x/net v0.20.0
 	golang.org/x/sys v0.16.0