Merge #12198: rpc: Add deprecation error for getinfo

Summary:
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

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

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3182