docs: [apm] link to master in n.x branches (#56538)

docs/Versions.asciidoc

@@ -73,3 +73,12 @@ Shared attribute values are pulled from elastic/docs
 ///////
 
 include::{asciidoc-dir}/../../shared/attributes.asciidoc[]
+
+///////
+APM does not build n.x documentation. Links from .x branches should point to master instead
+///////
+ifeval::["{source_branch}"=="7.x"]
+:apm-server-ref:       {apm-server-ref-m}
+:apm-server-ref-v:     {apm-server-ref-m}
+:apm-overview-ref-v:   {apm-overview-ref-m}
+endif::[]