Use `PATCH` instead of `PUT` for `transformEntries`

[Swissbite]

The current implementation uses the PUT methods to replace an entry on itself. But it falls short as soon as the content gets larger and a single entry exceeds the payload limit of 1 MB.

It would be nicer, if the migration itself would use the PATCH options introduced in the API (which is also used on the Contentful Web Client) to update an entry. That would reduce the payload and would not require to hack around with makeRequest for a migration.

[Niko-Berry-Contentful]

Hey. The team and I are investigating whether this makes sense to address through the client or if this is something better handled through the server changing the file size limit.