Clean up build system in preparation of SFML 2.6.0 and 3.0.0 releases

[ChrisThrasher]

I made a few changes that bring the CSFML build scripts more in line with what the SFML 2.6.x build scripts look like as well as a tweak to CI to update a deprecated CI environment we were still using. The only drastic change I may recommend we make is enabling compiler warnings as we did recently in SFML 2 and 3. I'm not sure how valuable that would be, but at least adding a few basic warnings would be a good idea.