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

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12035