go.mod: use go1.20 and import prometheus-operator from the openshift …

…fork

Since p-o v0.70.0 uses 1.21 and relies on 1.21 features we have some
downstream commits to make it 1.20 compatible.
We should revert this once we have 1.21 builder images.

Signed-off-by: Jan Fajerski <jfajersk@redhat.com>

go.mod

@@ -1,6 +1,6 @@
 module github.com/openshift/cluster-monitoring-operator
 
-go 1.21
+go 1.20
 
 require (
 	github.com/Jeffail/gabs v1.4.0
@@ -158,3 +158,5 @@ require (
 )
 
 replace k8s.io/kube-openapi => k8s.io/kube-openapi v0.0.0-20230928195430-ce36a0c3bb67
+
+replace github.com/prometheus-operator/prometheus-operator v0.70.0 => github.com/openshift/prometheus-operator v0.20.1-0.20231211152750-5bf04f3ca997