cmake: Update formatting and switch to native find_package call for Qt6

[PatTheMav]

Description

Cleans up the CMake build script to go with the changes introduced by obsproject/obs-studio#9769.

Motivation and Context

Reduce code complexity and makes CMake code easier to read in places.

How Has This Been Tested?

Tested on Windows 11, macOS 14, and Ubuntu 23.04.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.

[RytoEX]

Looks good to me.