Replace AppVeyor with focused GitHub Actions coverage

[SRombauts]

Summary

• remove the AppVeyor configuration, badge, build-system entry, documentation, and active CI guidance
• add one Visual Studio 2022 Release Win32/x86 entry to the GitHub Actions CMake matrix while retaining the latest-MSVC Debug job
• preserve historical AppVeyor changelog entries and document the retirement in the current unreleased section

Validation

• parsed .github/workflows/cmake.yml and asserted the requested Windows matrix coverage with no VS2019 entry
• configured and built a clean Release tree with tests and examples enabled
• ran CTest in Release mode: 67 unit tests and the example test passed
• searched the repository for AppVeyor; only historical and current-release CHANGELOG entries remain
• ran git diff --check master...HEAD

[coveralls]

coverage: 99.713%. remained the same — ci-replace-appveyor into master