rpc: Default rpcthreads value too small(4)

## Summary

Hello @wu-emma @van-orton

This issue aims at :-

- Increase the default number of RPC threads to 16 and remove outdated references to older Bitgesell Core versions.

- Contributors to feel free to flag any other inconsistencies you come across with the current defaults.

## Motivation

This Issue is motivated by this Bitcoin Core issue/implementation: [Default rpcthreads value (4) is too small ](https://github.com/bitcoin/bitcoin/issues/29386)

Start by locating the RPC thread default and references to older Bitgesell Core versions, using the linked Bitcoin Core issue for context. Done means the default is 16 and outdated references are removed; review any other default inconsistencies found during that search.

Written by the indexing model from the issue text.