Fix XRD field inconsistencies and update to crossplane-system namespace

[ytsarev]

Description of your changes

XRD Field Consistency

• Fix target field mismatch: status.servicePrincipalDetails → status.servicePrincipals (2 examples)
• Add missing status input fields to XRD: status.group.name, status.groups, status.users, status.servicePrincipalNames
• Improve field descriptions to clarify query result fields vs reference input fields

UXP v2 Namespace Update

• Update namespace: upbound-system → crossplane-system (30 occurrences)
• Updated across all example compositions, secrets, operations, and README

Validation

• ✅ All example field references now match XRD definitions
• ✅ All status references properly defined in XRD schema

E2E acceptance test

k get -f example/xr.yaml -o yaml
apiVersion: example.crossplane.io/v1
kind: XR
metadata:
  annotations:
...
 servicePrincipals:
  - appId: <redacted>
    description: ""
    displayName: yury-test
    id: <redacted>

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.