Fix references to system's "Integration Page" in Advanced Apicast Configuration doc page

The Advanced Apicast Configuration doc page contains multiple references to proxy configuration options on System that before 2.7 used to be available in the "Integration page".

The "Integration" section in System has changed significantly since 2.6 with some fields moved to "Integration > Configuration", for example, also removal of the "Update & Test Staging Configuration" button, etc. Nonetheless the documentation for Apicast was not adjusted accordingly.