frontend, shared: Remove code for Qt versions older than 6.10

[RytoEX]

Description

Remove code for Qt versions older than 6.10

Motivation and Context

Qt 6.10 is the minimum supported Qt version now.

Note that this does remove code that is modified in #13622. If it's preferable to simply leave that code untouched, then I'll just do that.

See:

• Dropping Support for Ubuntu 24.04, Qt 6.4, and FFmpeg 6.1 #13570

How Has This Been Tested?

Compiled and ran locally on Windows 11.

Types of changes

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

Checklist:

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

[tytan652]

It's missing removal in-cmake code, e.g.

obs-studio/frontend/cmake/os-linux.cmake

Lines 15 to 17 in a8b04ac

	if(Qt6_VERSION AND Qt6_VERSION VERSION_LESS "6.9.0")
	target_link_libraries(obs-studio PRIVATE Qt::GuiPrivate)
	endif()

[RytoEX]

It's missing removal in-cmake code, e.g.

obs-studio/frontend/cmake/os-linux.cmake

Lines 15 to 17 in a8b04ac

	if(Qt6_VERSION AND Qt6_VERSION VERSION_LESS "6.9.0")
	target_link_libraries(obs-studio PRIVATE Qt::GuiPrivate)
	endif()

Pushed a squash commit addressing this.

[tytan652]

os-freebsd.cmake had one too…

[RytoEX]

os-freebsd.cmake had one too…

Found and removed.

[tytan652]

LGTM