[mdatagen] add deprecation start date and migration instructions

[atoulme]

Component(s)

No response

Describe the issue you're reporting

When setting the status of a component to deprecated, mdatagen should validate the presence of two additional fields in metadata.yaml:

• One field that contains the date of the deprecation, in format YYYY-MM-DD ("2006-01-02" date parsing format)
• One field that contains a non empty string with migration instructions. This is a free form field. It is reasonable for someone to enter "No migration is offered for this component", and reviewers of the change can discuss.

Those fields should surface in the README.md generated text.