util: change GetWarnings parameter to bool

Summary:

GetWarnings() changes the format of the output warning string based on a
passed-in string argument that can be set to "gui" or "statusbar".
Change the argument to a bool verbose:

• there are only two types of behaviour, so a bool is a more natural

argument type

• changing the name to 'verbose' does not set any expectations for the

how the calling code will use the returned string (currently,
'statusbar' is used for RPC warnings, not a status bar)

• removes some error-handling code for when the passed-in string is not

one of the two strings expected.

This is a partial backport of Core PR17750 [2/3]
https://github.com/bitcoin/bitcoin/pull/17750/commits/492c6dc1e742a62599dc6d5ba6c3896825b5144f

Depends on D8329

Test Plan:
ninja && src/qt/bitcoin-qt -regtest

The warning banner "This is a pre-release test build..." uses getWarnings().

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8330