Deprecate Ubuntu 20.04 AppVeyor build

Hey @Try,

Ubuntu 20.04 has not been supported by Canonical (except the paid ESM-Version) since  May 2025 [1]. The build system should be revised for 24.04 (glslang-tools, for example.).

I'd be willing to take care of it, once #875  gets merged (or doesn't).

[1] https://ubuntu.com/about/release-cycle?product=ubuntu&release=ubuntu&version=20.04+LTS