Merge #14306: AppVeyor: Move AppVeyor YAML to dot-file-style YAML

ff40357 AppVeyor: Move AppVeyor YAML to dot-file-style YAML (Mitchell Cash)

Pull request description:

  AppVeyor supports dot-file-style YAML named `.appveyor.yml` as is. This helps keep the root of the repository clean(ish) and readable by having the CI files as dot-files.

  Source: https://www.appveyor.com/docs/build-configuration/#yaml-file-alternative-naming

Tree-SHA512: da2a78aff775e5e146f0784b1a6617d0371a5821da8a53be9e4aa57409cb16360f43d0afa5745f81f776599950cab4219a5d7ee7247d42e25861963ea487db66

Author: MarcoFalke