finance/kmymoney: Update to 5.2.1

Switch to Qt6/KF6. This is only the second release with KF6 support,
so there may be some remaining bugs.

Release notes for 5.2.0:
https://kmymoney.org/2025/06/21/kmymoney-5-2-0-released.html
Release notes for 5.2.1:
https://kmymoney.org/2025/08/04/kmymoney-5-2-1-released.html