Qt6 migrations that are compatible with Qt5

[mjcarroll]

Set of qt5+qt6 compatible changes to CMake and remove logic deprecated in qt < 5.15

[iche033]

tested on jammy with qt 5.15, opengl and vulkan on machine without a gpu and this is working fine for me.

As before, I still have issue launching gui with fuel_textured_mesh.sdf world in vulkan. It's an existing issue and not due to changes in this PR.

[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (91b50b8) 70.08% compared to head (dffd3a0) 70.15%.

❗ Current head dffd3a0 differs from pull request most recent head acb224b. Consider uploading reports for the commit acb224b to get more accurate results

Files	Patch %	Lines
src/plugins/minimal_scene/MinimalSceneRhiVulkan.cc	0.00%	9 Missing ⚠️
src/plugins/minimal_scene/MinimalScene.cc	65.21%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #598      +/-   ##
==========================================
+ Coverage   70.08%   70.15%   +0.07%     
==========================================
  Files          38       38              
  Lines        5348     5345       -3     
==========================================
+ Hits         3748     3750       +2     
+ Misses       1600     1595       -5     

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