Fix Qt6 detection in CMakeLists.txt by prioritizing qmake6 and fixing…

[Aram-Vn]

Fix Qt6 detection in CMakeLists.txt

Description

This PR improves Qt6 detection in CMakeLists.txt by ensuring that qmake6 is prioritized over qmake and by fixing version comparison logic.

Prioritizes qmake6 over qmake to avoid selecting Qt5.
Adds explicit search paths for Qt6 (/usr/lib/qt6/bin, /usr/bin) to ensure correct detection.
Fixes version comparison by using VERSION_LESS instead of LESS.
Removes trailing whitespace from QT_VERSION to avoid incorrect parsing.

Test Steps

• Ensure both Qt5 and Qt6 are installed (qmake --version and qmake6 --version).
• Run cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug and cmake --build build --config Debug
• Verify that Qt6 is selected (Using Qt version 6.x.x should appear).
• Build QGroundControl and check for successful compilation.

Checklist:

• Review Contribution Guidelines.
• Review Code of Conduct.
• I have tested my changes.

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