Bump KF5 version requirement to 5.45
Summary: Discover depends on a feature in Kirigami (D11197) that was/is being introduced in 5.45. This patch marks that dependency in CMake.
Test Plan: Still compiles when KF 5.45 is installed; doesn't when it isn't.
Reviewers: apol, Discover Software Store
Reviewed By: apol, Discover Software Store
Differential Revision: https://phabricator.kde.org/D12035