PR12198 backport https://github.com/bitcoin/bitcoin/pull/12198/files
49e5f3f rpc: Add deprecation error for getinfo (Wladimir J. van der Laan)

Pull request description:

Add a short informative deprecation message when users use `getinfo`, that points them to the new calls
 here to get the different information fields.
This is meant to be temporary, for one release only.

Depends on D3120

make check
test_runner.py
Try it out:
bitcoin-cli getinfo
bitcoin-cli -getinfo