Add migration guide for Source.queue users

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.

Start with the migration guide at the linked Pekko documentation page and the Source.queue deprecation text in the issue. Add concrete examples for the specified OverflowStrategy.backpressure alternatives, then verify the guide clearly explains the migration for affected pekko-connectors code.

Written by the indexing model from the issue text.