Queries for `is_control=False` no longer work with restriction against unbound variables

This is really only a problem for the interactive docs, where setting is_control to False is an option.

See also this note:

https://github.com/neurobagel/api/blob/0bea618aec64dda22df43378236bbf2a30793704/app/api/utility.py#L191-L194

hasSubjectGroup can never be != IS_CONTROL_TERM because the only possible subject group at the moment is healthy control. Otherwise, a subject would not have this property in the first place. So allowing users to query for a value of False will always return no results, violating the user expectations.

TODO:

[x] no longer allow False as a query value for is_control
[x] update node API
[ ] also update federation API (possibly additional issue for that)

neurobagel / api

Queries for `is_control=False` no longer work with restriction against unbound variables #332