[DOCS] Fixes broken links to APM 7.x

Author: lcawl

docs/en/install-upgrade/breaking.asciidoc

@@ -4,7 +4,7 @@
 Before you upgrade, you must review the breaking changes for each product you
 use and make the necessary changes so your code is compatible with {version}. 
 
-** {apm-server-ref-70}/breaking-changes.html[APM Server {version} breaking changes]
+** {apm-server-ref-70}/breaking-changes.html[APM Server 7.0 breaking changes]
 ** {beats-ref}/breaking-changes.html[Beats {version} breaking changes]
 ** <<elasticsearch-breaking-changes,{es} {version} breaking changes>>
 ** {hadoop-ref}/breaking-changes.html[Elasticsearch Hadoop {version} breaking changes]

docs/en/install-upgrade/upgrading-stack.asciidoc

@@ -64,7 +64,7 @@ on the cluster during the upgrade process. For more information, see
 .. Kibana: {kibana-ref}/upgrade.html[upgrade instructions]
 .. Logstash: {logstash-ref}/upgrading-logstash.html[upgrade instructions]
 .. Beats: {beats-ref}/upgrading.html[upgrade instructions]
-.. APM Server: {apm-server-ref}/upgrading.html[upgrade instructions]
+.. APM Server: {apm-server-ref-70}/upgrading.html[upgrade instructions]
 
 NOTE: Logstash 6.7 and Beats 6.7 are compatible with all 7.x versions of
 {es}. This provides flexibility in when you schedule the upgrades