[docs]: invalid GitHub actions badge URLs in readme

# Documentation request <!-- Please include affected doc URL, command line utility or tool --> The three workflow badges in the [README ](https://github.com/canonical/cloud-init/blob/main/README.md)are broken because the linked workflow files don't exist: - unit.yml - integration.yml - check_format.yml These need to be updated to point to the actual workflow files in the .github/workflows/ directory, or removed if no longer needed