ci: Add Github Actions support

[Xaymar]

Description

Replaces Jenkins and Appveyor with Github Actions, while also upgrading certain internal code to work with newer compilers and CMake versions. Additionally strips out some completely unused files from the repo, which were leftovers from early attempts at things.

Motivation and Context

Jenkins and Appveyor are no longer active for this repo.

How Has This Been Tested?

Builds on my machine™ and Github Actions.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)
• Documentation (a change to documentation pages)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.

[Xaymar]

Ready to merge now! Accidentally had my own origin pointing at a different repository, so my rebase to master wasn't actually a rebase to master.

[Xaymar]

Closing as requested.