Add migration guide for Source.queue users

[pjfanning]

https://pekko.apache.org/docs/pekko/2.0/migration/migration-guide-1.x-2.x.html

2.0.0-M4 deprecated Source.queue with an OverflowStrategy. In particular the OverflowStrategy.backpressure alternatives should be specified.

The deprecation is a big issue for pekko-connectors which has a lot of affected code.

  @deprecated(
    "Prefer Source.queue(bufferSize) which materializes a BoundedSourceQueue with synchronous feedback " +
    "(dropping the newest element when the buffer is full). For backpressure, use Source.actorRefWithBackpressure " +
    "or MergeHub.source instead. See the Pekko Streams documentation for the Source.queue migration guide.",
    since = "2.0.0")

But we need actual code examples.