Drop appveyor badge (#372)

* Drop appveyor badge

* add CI badge alt label

README.md

@@ -10,8 +10,8 @@
     <a href="https://buildkite.com/julialang/geophysicalflows-dot-jl">
         <img alt="Buildkite CPU+GPU build status" src="https://img.shields.io/buildkite/4d921fc17b95341ea5477fb62df0e6d9364b61b154e050a123/main?logo=buildkite&label=Buildkite%20CPU%2BGPU">
     </a>
-    <a href="https://ci.appveyor.com/project/navidcy/geophysicalflows-jl">
-        <img alt="Build Status for Window" src="https://img.shields.io/appveyor/ci/navidcy/geophysicalflows-jl/main?label=Window&logo=appveyor&logoColor=white">
+    <a href="https://github.com/FourierFlows/GeophysicalFlows.jl/actions/workflows/CI.yml">
+        <img alt="CI Status" src="https://github.com/FourierFlows/GeophysicalFlows.jl/actions/workflows/CI.yml/badge.svg">
     </a>
     <a href="https://FourierFlows.github.io/GeophysicalFlowsDocumentation/stable">
         <img alt="stable docs" src="https://img.shields.io/badge/documentation-stable%20release-blue">