docs: Add CRD cleanup warning to uninstall guides across all versions

[u-kai]

Problem

Users expect helm uninstall crossplane to completely remove Crossplane, but CRDs with *.crossplane.io suffix remain in the cluster.
This can cause confusion.

Solution

Added consistent warning sections explaining:

• CRDs aren't automatically removed by helm uninstall
• How to view remaining CRDs with kubectl get crd | grep crossplane.io
• Safe manual deletion example
• Important data loss warning

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[jeanduplessis]

Thank you for this contribution @u-kai