unknown flag: --key when use syft attest --key [KEY]

[deblaci]

What happened:
Local private key support is not working because --key option is not exist anymore.

According to doc:

To generate an SBOM attestation for a container image using a local private key:
syft attest --output [FORMAT] --key [KEY] [SOURCE] [flags]

What you expected to happen:
Generate a packaged-based SBOM

Steps to reproduce the issue:
syft attest --output cyclonedx-json --key cosign.key ubuntu:latest

Anything else we need to know?:

Environment:

• Output of syft version: 0.101.1
• OS (e.g: cat /etc/os-release or similar): macOS 14.3