Fix checking if connectors are included by a defined constant instead of a get_included_files() check.

This resolves an edge case scenario where a file that is automatically prepended (e.g. auto_prepend_file) causes the included check to fail as the connector.php is not the first file in get_included_files(). The entire manager becomes unusable in this case, as none of the AJAX requests work as expected.

The new check per this commit first checks the existence of a constant (which the other connectors define). As alternative, it also checks if the MODX_CORE_PATH is defined, which is a pretty good indicator that for example a third party connector loaded the main connector (as they can't find the connector location, without knowing the location of the core and loading the config file).
Update RSS Feed link to a new one that only contains Revo topics
[rebuild] Updated RSS security feed to be Revo specific

* upstream/pr/11770:
  Update RSS Feed link to a new one that only contains Revo topics
Refactor validation of a connector being included

Resolves #11738

Merge remote-tracking branch 'origin/pr/11748'

* origin/pr/11748:
  Fix checking if connectors are included by a defined constant instead of a get_included_files() check.