Migrations fail if 'Require explicit user-specified values' is set on an org/enterprise custom property

We have a custom property named `owner` which is mandatory for all our repos. The `Require explicit user-specified values` setting is enabled on this property which is fine normally - you just supply the value during creation in the API call. There is no way to do this for a migration though - the `gh ado2gh migrate-repo` will just fail.

The migration command should support setting custom properties for the new repo, because otherwise the alternative is to disable the requirement every time you do a migration (and then you have to ensure that nobody sneaks in a new repo in that time and forgets to set the property).