Merge #10858: [RPC] Add "warnings" field to getblockchaininfo and unify "warnings" field in get*info RPCs

Summary:
395cef7 Change getmininginfo errors field to warnings (Andrew Chow)
8502b20 Unify help text for GetWarnings output in get*info RPCs (Andrew Chow)
f77f0e4 Add warnings field to getblockchaininfo (Andrew Chow)

Pull request description:

The `getblockchaininfo` output does not contain the `warnings` field which the `getinfo`, `getmininginfo`, and `getnetworkinfo` RPCs have. It should have it as the errors pertain to the blockchain. This PR adds that field.

This PR also unifies the help text for the `warnings` field and its output position so that all of the `get*info` commands are consistent.

`getnetworkinfo`'s `errors` field is named `warnings`. I did not change this even though it is inconsistent since this naming has been in use for a long time.

Tree-SHA512: 385ab6acfee67fc8816f4d51ab2bd7a623264c7973906dfbab0a171f199e9db16fde19093a5bc3dfbdd4ff5f19d2186b646eb6b3bae0a4d7c9add43650a4a9d9

Backport of PR10858
https://github.com/bitcoin/bitcoin/commit/9a8e9167f2636fdc2fc047dfed1747920b0f047f?diff=unified#diff-a0c8f511d90e83aa9b5857e819ced344R1165

Completes T567

Test Plan:
make check
test_runner.py
getblockchaininfo should also produce a "warnings" field
getmininginfo should have a "warnings" field in place of its "errors" field during regular use
getmininginfo should behave as previously (i.e. have an "errors" field) when running bitcoind -deprecatedrpc=getmininginfo

Reviewers: deadalnix, jasonbcox, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2730