[keyserver] Only create robotext for relationship actions if createRobotextInThinThread is true

Summary:
This diff updates updateRelationships to use the new createRobotextInThinThread field introduced as part of the new API. It addresses ENG-9331.

Depends on D13438

Test Plan: I confirmed that this flag works in the correct cases as part of my testing for ENG-9269

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13439