Improve the migrate command based on the feedback

[DimitarTachev]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

1. useLegacyWorkflow flag is removed from the nsconfig (we do not use it anymore).
2. The code behind tns resources update is executed (it's a project migration).
3. A warning is shown if we remove nativescript-dev-less (we have a feature request for out of the box LESS CSS support with Bundle).

Related to: #4646