feat: removal of custom core group

[makdeniss]

Core API group simplification: removed the custom "core" group. Core resources are now accessed directly by version (e.g., v1) for queries/mutations, and core subscription names are flattened to start with the version (e.g., v1_configmaps).

Change Log

• 🔥 (breaking) Queries/Mutations: Path changed from core { v1 { ... } } to v1 { ... }. Update client queries accordingly.
• 🔥 (breaking) Subscriptions: Renamed from core_v1_<resource> to v1_<resource> (e.g., core_v1_configmaps → v1_configmaps).
• Docs: Updated docs/breaking-changes.md, docs/configmap_queries.md, docs/pod_queries.md, docs/quickstart.md, and docs/subscriptions.md to reflect the new core paths and subscription names.
• Tests: Adjusted standard Kubernetes tests to use the new core paths/names.