[Segment Replication + Remote Storage] Remove implicit replication strategy change when configuring remote storage.

[mch2]

Is your feature request related to a problem? Please describe.
Today when enabling remote storage for an index segrep will be enabled behind the scenes. This is misleading as user is not expecting to change replication strategy when configuring remote storage. There is a current limitation that remote storage is not supported with docrep, but could be in the future and would require reverting this coupling.

Describe the solution you'd like
Require replication strategy to explicitly be set as segment (either via cluster setting or in req) for remote storage to be enabled. Throw an exception if docrep is on the index.

Describe alternatives you've considered
Leave as is.

Additional context
None