Clarify all macOS versions from 12 are supported

Author: eregon

README.md

@@ -47,7 +47,7 @@ The action works on these [GitHub-hosted runners](https://docs.github.com/en/act
 | Operating System | Supported |
 | ---------------- | --------- |
 | Ubuntu  | `ubuntu-20.04`, `ubuntu-22.04`, `ubuntu-24.04` |
-| macOS   | `macos-12`, `macos-13`, `macos-14` |
+| macOS   | `macos-12+` |
 | Windows | `windows-2019`, `windows-2022` |
 
 The prebuilt releases are generated by [ruby-builder](https://github.com/ruby/ruby-builder)