CI for TIM-VX backend
* github actions for TIM-VX backend https://github.com/opencv/opencv/pull/21036
* add reference to yuentau/ocv_ubuntu:20.04; remove extra quotes; enable BUILD_TESTS
* rename to timvx_backend_tests.yml
* add image source prefix
* remove if condition for x86_64 simulator
changed OpenCV license from BSD to Apache 2 license
* as discussed and announced earlier, changed OpenCV license from BSD to Apache 2. Many files still contain old-style copyrights though
* changed wording a bit; preserve the original OpenCV BSD license