Minimum go version should be 1.21

Readme says minimum go version is 1.0, but with Go 1.20 go build throws:

`invalid go version '1.22.0': must match format 1.23`

because the go.work file has a patch version. Upgrading to 1.21 works. From https://go.dev/doc/go1.21:

`To improve forwards compatibility, Go 1.21 now reads the go line in a go.work or go.mod file as a strict minimum requirement: go 1.21.0 means that the workspace or module cannot be used with Go 1.20 or with Go 1.21rc1`