keepass: change from mono40 to mono54

Issue #31859 (https://github.com/NixOS/nixpkgs/issues/31859) highlighted TLSv1.1 / TLSv1.2
support isn't available with older versions of Mono 4.  This change moves to use Mono 5.4.  Tested as working with Keepass v2.38 for which PR https://github.com/NixOS/nixpkgs/pull/34360 is already open.