[TASK] Deprecate reachability.hpp

[tarang-jain]

• reachability.hpp was supposed to be deprecated in [FEA] Build Single Linkage API #820, but it seems cuml hdbscan tests (soft clustering) are currently using mutual_reachability_graph to compute core distances. reachability.hpp should be deleted once brute force and MR graph becomes part of all_neighbors API
• In general, it seems cuVS is lacking a public API to build a brute force all neighbors graph and slice the k-th neighbor (the only option right now is to build a brute force index). These are useful utilities, which should be exposed. We can consider exposing these two as separate APIs (brute force all neighbors graph and k-th neighbor slicing) so use-cases such as computing core distances or in general fetching the k-th neighbor for each vector do not need to be output the entire MR graph.

[jinsolp]

Suggested order:

• make HDBSCAN use all_neighbors mutual reachability
• generic core_dist function (e.g. slicing a certain column)
• remove rechability.hpp and related code paths