Fix async generateMetadata type in ts plugin (#46596)
Corrects async return type for `generateMetadata`
x-ref: https://github.com/vercel/next.js/pull/45723/files#r1121140364
## Bug
- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
