Switch to multi-group Kubebuilder layout

[afritzler]

Summary

Convert the project from the default single-group Kubebuilder layout to a multi-group layout so additional API groups can be added cleanly alongside the existing maintenance.metal.ironcore.dev group.

Motivation

Today all APIs live under api/v1alpha1/ and all controllers under internal/controller/. The project is expected to grow beyond the current Console CRD and the maintenance group, and adding more groups under the single-group layout would mix unrelated types and reconcilers in the same directories.

Proposed Changes

1. Enable multi-group mode:

   kubebuilder edit --multigroup=true

2. Move the existing API and controller into group-scoped directories:
   • api/v1alpha1/ → api/maintenance/v1alpha1/
   • internal/controller/ → internal/controller/maintenance/
3. Update import paths in all Go files (cmd/main.go, controller, tests, etc.).
4. Update the path field for the Console resource in PROJECT.
5. Fix relative CRD paths in the test suite (suite_test.go needs one additional ..).
6. Regenerate manifests and deepcopy:

   make manifests generate

7. Verify everything still builds and tests pass:

   make lint-fix test

Acceptance Criteria

• PROJECT contains multigroup: true and the updated resource path.
• Console types live under api/maintenance/v1alpha1/ and its controller under internal/controller/maintenance/.
• make manifests generate produces no diff after the move.
• make test passes.
• No functional change to the Console CRD or reconciler behavior.

Out of Scope

• Adding any new API group or kind. This issue is purely the layout change.