fix(event_manager): Resolve TypeError while recording first insight span

[jan-auer]

Fixes a regression introduced in #86604 that resulted in a captured
TypeError when recording the first insight span for a project.

We switched from using signals, which are prohibited in
EventManager.save, to directly calling the onboarding receiver
function. In this specific instance, we missed that the function was
decorated, and the import resulted in None. For an unknown reason, the
resulting TypeError did not show up in Sentry.

Projects that have not recorded these flags since the regression was
rolled out will be flagged when they ingest the next span of the
corresponding insights module.