fix: implement signature verification for jx upgrade and update k8s docs

[Mallikarjunadevops]

This PR implements signature verification for the Jenkins X CLI during the upgrade process. It also updates the README with supported Kubernetes versions.

Changes:

• Signature Verification: Added a new pkg/util/verify helper to verify Cosign-compatible ECDSA signatures.
• jx upgrade cli: Now downloads and verifies the signature of the binary before applying the upgrade.
• jx upgrade plugins: Added hooks for plugin signature verification.
• Documentation: Updated README with supported K8s versions 1.30-1.32.

Related issues: #8438, #8325

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Hi @Mallikarjunadevops. Thanks for your PR.

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