[FEA] Remove deprecated raft::neighbors::experimental::cagra namespace

[cjnolet]

PR #1666 officially promotes CAGRA from experimental by renaming all of the raft::neighbors::experimental::cagra namespaces to raft::neighbors::cagra. To keep code backwards compatible (eg for existing users), we're keeping the cagra experimental namespace around for a couple releases before removing it altogether. This issue is a placeholder to remove the experimental namespace in 23.12.

The CAGRA googletests will also need to be updated to use raft::neighbors::cagra instead of the experimental namespace. Keeping them using experimental for now to test backwards compatibility.