Use API database group-project association instead of keycloak attributes

[shreddedbacon]

Now that uselagoon/lagoon#3612 has been merged, opensearch-sync will need to be updated to consume the group-project associations the same way that the API does as the attributes in keycloak will be stop being populated in a future release of Lagoon.

This means that this support is not required to be done in time for the Lagoon v2.18.0 release, but should be usable prior to v2.20.0 when the attributes will likely be removed.