Rename master to main for APM Server project

Author: v1v

conf.yaml

@@ -1214,8 +1214,8 @@ contents:
                 prefix:     guide
                 index:      docs/integrations-index.asciidoc
                 current:    *stackcurrent
-                branches:   [ master, 8.0, 7.17, 7.16 ]
-                live:       *stacklive
+                branches:   [ {main: master}, 8.0, 7.17, 7.16 ]
+                live:       *stacklivemain
                 chunk:      2
                 tags:       APM Guide
                 subject:    APM