Fix migration of group config from old location to new location

On Windows, we cannot use QStandardPaths::GenericConfigLocation because
Gpg4win 5 changes this path from Qt's standard (%LOCALAPPDATA%) to
%GNUPGHOME%/kleopatra. We need to look for the group config in
%LOCALAPPDATA%.

(cherry picked from commit 4ec9831859c44014c84cbf925b35193ef1548da2)

• GnuPG-bug-id: T7574