Validation: Check for deprecated license identifiers

[Alexander-Wilms]

Another check that could be added to appstreamcli validate is whether the metadata_license and project_license identifiers are deprecated using the isDeprecatedLicenseId field in the licenses.json.

[ximion]

Yeah, SPDX has changed this so many times though that I am a bit burned by having to play catch-up with license name changes, and I don't want to inflict the rename dance onto others as well.

That said, SPDX has been stable the past years, so I wouldn't oppose if some INFO severity hint was added for this :-)