Geo Sites - Update API endpoints to use geo_sites

What / Why

Related to #369140
Follow up for thread: !119685 (comment 1389051615)

Now that geo_sites API endpoint is supported we should switch the internal implementations from geo_nodes => geo_sites.

Implementation plan

1. Update geo_nodes => geo_sites in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/assets/javascripts/api.js#L7-10