GA: Promote `GitRepository` to `source.toolkit.fluxcd.io/v1`

Items to be reviewed plus any outstanding work required to bump API to `v1`:
- [x] Documentation
- [x] API Validation
- [x] Test Coverage
- [x] Deprecate `spec.gitImplementation`
- [x] https://github.com/fluxcd/source-controller/issues/1024
