The custom fee input box now has a minimum value equal to the minimum
required fee. Before a value below the minimum fee could be entered
which was confusing since the minimum fee would still be paid even
though a lower amount was entered.
Backport of Core PR14608 [2/2]
Commit https://github.com/bitcoin/bitcoin/pull/14608/commits/a16f44c040642432e234ad7317b00fe829c2d9e7
Depends on D8176