[azure-messaging-eventhubs] In migration guide, we should document the reason for removal of `InMemoryCheckpointManager` and `InMemoryLeaseManager`

[XiaofeiCao]

Some of our Track 1 users are using InMemoryCheckpointManager and InMemoryLeaseManager:
e.g. https://github.com/logstash-plugins/logstash-input-azure_event_hubs/blob/main/.ci/integration/event_hub_consumer/src/main/java/com/microsoft/azure/eventprocessorhost/Consumer.java#L31-L39

We have dedicated code sample to use BlobCheckpointStore. We should also consider including the reason for the removal the Track 1's InMemory variant.
E.g. Quote from @srnagar

The removal of in-memory checkpoint store in Track 2 was a conscious decision as that led to real production issues in Track 1.

Rational be:

1. Missing Track 2 variant may confuse user during migration process
2. Guide user through InMemory -> Blob variant migration