Optimize `recentMilestones` field in `RepositoryNode`

**Is your feature request related to a problem? Please describe.**
When querying the recentMilestones 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

Start by locating RepositoryNode and its recentMilestones field, then read the linked Strawberry Django optimizer guidance on Prefetch. Verify the change removes the N+1 queries when recentMilestones is queried and preserves the field's results.

Written by the indexing model from the issue text.