Merge #15471: rpc/gui: Remove 'Unknown block versions being mined' warning

Summary:
PR15471 backport https://github.com/bitcoin/bitcoin/pull/15471/files
ef362f2773 rpc/gui: Remove 'Unknown block versions being mined' warning (Wladimir J. van der Laan)

Pull request description:

Due to miners inserting garbage into the version numbers causing false positives, the current version signalling has become completely useless. This removes the "unknown block versions" warning which has the tendency to scare users unnecessarily (and might get them to "update" to something bad).

It preserves the warning in the logs. Whether this is desirable can be a point of discussion.

Depends on D3230

Test Plan:
make check
test_runner.py

Reviewers: jasonbcox, Fabien, schancel, #bitcoin_abc, deadalnix, nakihito

Reviewed By: jasonbcox, Fabien, #bitcoin_abc, deadalnix, nakihito

Subscribers: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D3181