fix: upgrade go version -> 1.20

[behnambm]

the go.mod and readme files are not up to date.
Due to upgraded github.com/prometheus/common, ghz will only compile with version 1.20 or higher.

The compile time error(on go1.19.13 linux/amd64):

go: downloading cloud.google.com/go/compute v1.20.1
# github.com/prometheus/common/model
pkg/mod/github.com/prometheus/[email protected]/model/metric.go:364:33: undefined: strings.CutPrefix
note: module requires Go 1.20