Notifications: Change authority assignments

With this PR I would like to add Ingmar Szmais (@iszmais) to
the following authority sections of `components/ILIAS/Notifications`:

- `Authority to Sign off Conceptual Changes`
- `Authority to Sign off Code Change`
- `Authority to Curate Test Cases`

Ingmar provided most of the code changes for this
component in the last releases and already
did some conceptual work with/for interested parties
for ILIAS 10.