Optimize `issues` field in `RepositoryNode`

**Is your feature request related to a problem? Please describe.**
When querying the issues field, we get N+1 queries.

**Describe the solution you'd like**
The current optimization hint is not enough for optimizing the query. We should use Prefetch instead.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Are you going to work on implementing this?**

- [ ] Yes
- [x] No

**Additional context**
https://strawberry.rocks/docs/django/guide/optimizer#optimization-hints