Add a flag to `renovate-config-validator` to auto-migrate a file

[jamietanna]

In the case that Renovate detects that there is a Config Migration necessary, running renovate-config-validator will log i.e.:

WARN: Config migration necessary
       "oldConfig": {
         "$schema": "https://docs.renovatebot.com/renovate-schema.json",
         ...
       },
       "newConfig": {
         "$schema": "https://docs.renovatebot.com/renovate-schema.json",
         ...
       }

It's possible to copy-paste the output from newConfig into your configuration file, but it's a little awkward.

We could provide a first-class means to --fix/--migrate the configuration file if we find any issues, to make it easier for folks to auto-migrate their configuration.

(This also has issues when using i.e. JSON5, where comments are lost - which AFAIK, is still an issue with automated config migration PRs, too)