Update CI compiler version

[juntuu]

Everything should already compile just fine with gcc 10+ or clang 11+ (#190), which are mentioned in the readme.
So, the CI compiler version should probably be updated, as it doesn't even have reasonable c++17 support.

What would be a realistic timeframe for upgrading the CI compiler?

At the moment not supported for example:

• std::reduce() (Port conversions to c++ #200)
• <filesystem> (Fix handling of .. in jepath() (requires <filesystem> header) #170)

[Sebanisu]

https://github.com/codereport/An-Algorithm-Library/blob/3ac57bd561b46ffa1dca9047961ed909b06cecd2/appveyor.yml

This could appveyor.yml could be dropped into j as well. I got it working with gcc 10.2, clang 12 and msvc. At least it's running the tests I made in my pull request on all 3. There could be improvements made. I kinda stopped when I got something that works. :P