Update CI workflows

[remia]

As discussed during TSC meetings, here are some update and new CI workflows.

This PR contains the following changes:

• Remove VFX 2019 platform testing from the CI workflow as well as some redondant jobs, there are 3 slots left (out of the 20 for concurrent processing) for the upcoming VFX2023 images.
• Consequently remove Python2 from the officially supported Python versions, it is no longer tested and advertised in the package metadata but I didn't actively break it intentionally (ie. compatibility conditional paths are still here).
• Extract "latest dependencies" jobs from the nightly analysis into their own workflow, note this workflow is still failing at the moment.
• Add a new "platform latest" nightly build using latest runners available on Github, C++20 standard and compiler sanitizers (only address enabled at this time). This detected a few memory out of bounds and leaks issues in apphelper and unit test, nothing in the core library so far.
• Update to CMake 3.13 minimum version required to use LINK_OPTIONS and bumped Imath external project build to 3.1.6.

Note some jobs changed their names in the CI workflow, updating the required list is again going to be needed, hopefully this will happen less in the future as I removed some variability from these name like compiler versions which are not always fixed even in ci-20xx docker images (Clang). I also had a recurring issue where the GCC based linux platform latest build was killed by GitHub Action during compilation of OpenEXR with no diagnostic messages, apparently this can happen when some hardware resources limits are reached. This was on my fork but may be different here, this could require further investigations, usually manual relaunch of the failed jobs fix the issue.

[remia]

Chatting with @JeanChristopheMorinPerso, I disabled parallel build on OpenEXR ExternalProject, I'll try to trigger the build a couple of times but on the first try it seemed to have resolved the issue!

[doug-walker]

For anyone else reviewing the new CI settings, I made a spreadsheet showing the current and new matrix: https://docs.google.com/spreadsheets/d/1Iw5uRw6voB54wFpkSSOGrcAXkciZfx9nlBSVHz6N4P8/edit?usp=sharing

[remia]

Thanks for the review @doug-walker, most of the points are about the old analysis workflow (new dependencies_latest) which I didn't touch and is currently failing in nightly for the past few months. From my understanding @cedrik-fuoco-adsk was about to work on these so I think it's best to push the comments for that upcoming work? I'm not really confortable updating things in there knowing the builds are going to fail early and I won't be able to test.