emulators/aqemu: Update to 0.9.6.g20250311, migrate to Qt6, take maintainership

Upstream aqemu has been abandoned since 2017 and is stuck on Qt5. Switch
the source to the maintained Qt6 fork at https://github.com/AQEMU/aqemu.

• Raise the compiler floor to C++17
• Bump the CMake floor to 3.16 for minimum Qt6 requirement and CMake 4 compatibility
• Extend the Linux-only install rules to FreeBSD
• Fix the Get_Free_Memory_Size signature for the FreeBSD build
• Drop deprecation

Approved by: arrowd (co-mentor)