Outdated Go version recommended in README

[thayol]

While running $ make build I encountered the following problems:

# github.com/prometheus/common/model
note: module requires Go 1.20
# golang.org/x/net/http2
note: module requires Go 1.18
# github.com/gin-gonic/gin/render
note: module requires Go 1.20
# github.com/gin-gonic/gin/binding
note: module requires Go 1.20

The current README says Go Version >= 1.16 is sufficient but I tried with 1.17.4 and it failed.

Upgrading to the latest version solved the issue but the README should recommend at least 1.20 based on the errors I received.