Update migration banner URL to /migration

[alexandersandberg]

Motivation:

The banner currently links to https://www.swift.org/migration/documentation/migrationguide/, but the migration guide will soon live at https://www.swift.org/migration/ instead, and we can link to it directly.

https://www.swift.org/migration/ currently does redirect to the migration guide but with a redirection message—which is why we didn't want to use that URL immediately.

This PR is just a draft PR in preparation for when the redirect situation is fixed. Once that's done, this can be merged.

Modifications:

• Updated migration banner URL to https://www.swift.org/migration/

Result:

Nice and short URL!

[alexandersandberg]

@swift-ci please test