Crossplane ships CLI pre-flight check for v2 upgrades

Crossplane v2 is designed for backward compatibility, but removes a few v1 features like native patch-and-transform Compositions and ControllerConfigs. The new `crossplane beta upgrade check` command, available in the v1.20 CLI, performs a read-only scan of a live v1.x cluster to detect any breaking changes. It reports specific resources, explains the issue, and provides exact remediation steps—including conversion commands—so teams can upgrade with confidence. The tool exits non-zero when blockers exist and supports JSON output for automation in CI/CD pipelines.