Remove Ubuntu build workflow

[ranisalt]

Pull Request Prelude

• [x] I have followed proper The Forgotten Server code styling.
• [x] I have read and understood the contribution guidelines before making this PR.
• [x] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Ubuntu is outdated by definition, which prevents us from adopting updated versions of compilers and dependencies. There isn't much added value by testing with Ubuntu anyway, and the vcpkg workflow covers building and testing already.

vcpkg is well adopted in the community, and has been supported for years.

This does not mean Ubuntu is not supported: it is still officially supported, just like other common distros, as long as we have documentation about it in the wiki.

[DSpeichert]

That workflow was providing CodeQL reporting, so I guess that needs to be moved to a different workflow now.