GH Actions: Remove Linux distributions no longer supported as of 2021-10

[stephengtuggy]

Thank you for submitting a pull request and becoming a contributor to the Vega Strike Core Engine.

Please answer the following:

Code Changes:

• Have the PR Validation Tests been run? See https://github.com/vegastrike/Vega-Strike-Engine-Source/wiki/Pull-Request-Validation
• This is a documentation change only
• This is a CI / CD / build system change only

Issues:

• Please list any related issues
• 2021-09 Revisit Compiler/Platform Support #558

Purpose:

• What is this pull request trying to do? Discontinue building several Linux distributions that are no longer supported as of October 2021
• What release is this for? 0.9.x; another PR for 0.8.x pending
• Is there a project or milestone we should apply this to? 0.9.x; 0.8.x pending