[backport#14879]Add warning messages to the debug window

Summary:
Warning messages about using a pre-release test build was only displayed
when wallet was enabled. This added the warning also to the debug window
displayed in -disablewallet mode

Backport of Core PR14879

Test Plan:
ninja && ninja check
src/qt/bitcoin-qt -disablewallet

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7917