[FEAT shoot-grafter] - Allow targeting multiple NS per CI

### Priority

None

### Description

Currently we need to create one CareInstruction per targeted namespace.
We want to allow an array of targeted garden-namespaces with one CI.

## Acceptance Criteria

- [ ] field `gardenNamespace` is deprecated
- [ ] field `gardenNamespaces` is introduced.
- [ ] tests are in place to validate the behaviour

### Reference Issues

_No response_