[keyserver] Update usage of threads.default_role in thread-updaters.js

Summary:
We need to get rid of usages of threads.default_role throughout the codebase so we can eventually drop the column.

This diff handles the usage in thread-updaters.js

Depends on D9976

Addresses part of ENG-5833

Test Plan:
Tested both use cases of threads.default_role separately in this file:

• Attempts to fetch the role permissions for the default role in the thread (expected return result size is one row)
  • Ran the query before and after in my MariaDB on my local database and saw the same results printed out
• Fetches the user, role and default_role from memberships to determine whether the user is a 'default role user'
  • Ran the query before and after in my MariaDB on my local database and saw the same results printed out

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9977