Rationale

Go 1.21 is the current stable implementation, and will be the lowest supported version once Go 1.22 releases in Q1 2024.

Implementation

The quic-go dependency would need to be updated from 0.33.0 to something more recent that supports Go 1.21, e.g. 0.37.3

Error message when building with Go 1.21 currently:

21.00 # github.com/quic-go/quic-go/internal/qtls
21.00 /go/pkg/mod/github.com/quic-go/quic-go@v0.33.0/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

quic-go from 0.37 on no longer requires its own TLS code, which means it should survive updates to the Go compiler far more easily:

"With the upcoming Go 1.21 release, we're now able to rely on the Go standard library's TLS implementation's QUIC support."

https://github.com/quic-go/quic-go/releases/tag/v0.37.0