Remove the dead AppVeyor CI configuration

[tas50]

AppVeyor has not run for this repository in years, and the config could not run even if something did trigger it:

• image: Visual Studio 2019 is a retired build image
• Ruby 2.7 and 3.0 are below the gemspec's required_ruby_version >= 3.1
• branches: only: master never matches — the default branch is main
• clone_folder: c:\projects\winrm-fs predates the rename to chef-winrm-fs

Also removed:

• the AppVeyor status badge in the README, which pointed at the pre-fork winrb/winrm-fs project
• the appveyor.yml line from the repo tree in .github/copilot-instructions.md

Note on integration tests

AppVeyor was the only CI that ever ran rake integration, so the 305 lines of integration specs in spec/integration/ are now unreachable from CI. They were already unreachable in practice — nothing has run them since the branch filter stopped matching. Wiring rake integration up to a Windows GitHub Actions runner (which can Enable-PSRemoting and test against localhost, exactly as AppVeyor did) is worth doing as a follow-up; spec_helper.rb already reads the winrm_endpoint / winrm_user / winrm_password env vars needed for it.

The README also still carries a dead travis-ci.org badge and a winrm-fs gem-version badge, both pointing at the pre-fork project. Left alone here to keep this PR to one topic.