Harmonize naming of session repositories

[vpavic]

In Spring Session core modules we recently harmonized naming of session repositories - see spring-projects/spring-session#1455. This was triggered by addition of another Redis-backed SessionRepository implementation for 2.1 so we wanted that implementation name more closely reflect the concrete SessionRepository interface they implement.

To align with the above, Spring Session Data MongoDB should:

• rename MongoOperationsSessionRepository to MongoIndexedSessionRepository
• rename ReactiveMongoOperationsSessionRepository to ReactiveMongoSessionRepository

See spring-projects/spring-session@8cc8fbb for inspiration and strategy taken for preserving backwards compatibility.