Switch to sftp as default

[chris-rock]

Is your feature request related to a problem? Please describe.

cnspec uses scp as opt in and it is not the default behavior.

Describe the solution you'd like

Remove the use_sftp

packer-plugin-cnspec/.web-docs/components/provisioner/cnspec/README.md

Lines 42 to 43 in 6d19951

	- `use_sftp` (bool) - packer's SFTP proxy is not reliable on some unix/linux systems,
	therefore we recommend to use scp as default for packer proxy

option and make it the default behavior. To ensure smooth migration, we should set the value to true first and deprecate the field.

Describe alternatives you've considered

none

Additional context

We've seen scp communication fail, specifically with Windows 2025 and SSH.