chore: clean out some mentions of appveyor (#990)

.github/CONTRIBUTING.md

@@ -82,7 +82,7 @@ ones in all-contributors if any overlap.
 Steps:
 
 - Update changelog if needed
-- Update the version in `.appveyor.yml` and `include/CLI/Version.hpp`.
+- Update the version in `include/CLI/Version.hpp`.
 - Find and replace in README (new minor/major release only):
   - Replace " 🆕" and "🆕 " with "" (ignores the description line)
   - Check for `\/\/$` (vi syntax) to catch leftover `// 🆕`

CMakeLists.txt

@@ -227,7 +227,6 @@ if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
       /.travis.yml
       .swp
       /.all-contributorsrc
-      /.appveyor.yml
       /.pre-commit.*yaml)
 
   set(CPACK_DEBIAN_PACKAGE_ARCHITECTURE "all")

README.md

@@ -4,7 +4,6 @@
 
 [![Build Status Azure][azure-badge]][azure]
 [![Actions Status][actions-badge]][actions-link]
-[![Build Status AppVeyor][appveyor-badge]][appveyor]
 [![Code Coverage][codecov-badge]][codecov]
 [![Codacy Badge][codacy-badge]][codacy-link]
 [![License: BSD][license-badge]](./LICENSE) [![DOI][doi-badge]][doi-link]
@@ -1783,9 +1782,6 @@ try! Feedback is always welcome.
 [actions-link]: https://github.com/CLIUtils/CLI11/actions
 [actions-badge]:
   https://github.com/CLIUtils/CLI11/actions/workflows/tests.yml/badge.svg
-[appveyor-badge]:
-  https://ci.appveyor.com/api/projects/status/82niaxpaa28dwbms/branch/main?svg=true
-[appveyor]: https://ci.appveyor.com/project/HenrySchreiner/cli11
 [repology-badge]: https://repology.org/badge/latest-versions/cli11.svg
 [repology]: https://repology.org/project/cli11/versions
 [codecov-badge]: