Use latest 1.20 go version when building horizon docker image from so…

…urce (#4836)

The quickstart image uses Dockerfile.dev to build horizon from source so we
should be using the latest 1.20 release of Go to build the binary so we
can benefit from security fixes

services/horizon/docker/Dockerfile.dev

@@ -1,4 +1,4 @@
-FROM golang:1.19.1 AS builder
+FROM golang:1.20 AS builder
 
 ARG VERSION="devel"
 WORKDIR /go/src/github.com/stellar/go