CI: Use Qt6 by default when available

[RytoEX]

Description

Use Qt6 by default when available. This applies to the Windows, macOS, and Ubuntu CI jobs.

Motivation and Context

As announced on May 14, we plan to use Qt6 for OBS 28 on platforms where it is available. CI and build scripts should default to Qt6 to encourage adoption of Qt6 by plugin maintainers. This will also help us during our beta and release candidate period.

As stated in the Discussion post, moving to Qt6 also allows us to stop maintaining backported patches, which reduces our maintenance burden.

How Has This Been Tested?

I've verified that CI runs on my fork build with Qt6 when available (Windows, macOS, Ubuntu 22.04) and Qt5 when not available (Ubuntu 20.04). Flatpak will be covered in #6847.
https://github.com/RytoEX/obs-studio/actions/runs/2768057341

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[SuslikV]

CI builds for Windows platform from this time will run only on Win8.1 and up (dependencies, that were updated here, relay on functions of dxgi library that are specific to newer OSes).

[RytoEX]

CI builds for Windows platform from this time will run only on Win8.1 and up (dependencies, that were updated here, relay on functions of dxgi library that are specific to newer OSes).

That's expected and was announced in the pinned May 14 Discussion Post, Plan to Upgrade OBS Studio to Qt 6.