doc: update README with Actions status badge

and remove the appveyor and concourse badges

README.md

@@ -5,8 +5,7 @@ renamed to `mini_portile2`. For mini_portile versions 0.6.x and
 previous, please visit
 [the v0.6.x branch](https://github.com/flavorjones/mini_portile/tree/v0.6.x).
 
-[![Concourse CI](https://ci.nokogiri.org/api/v1/teams/nokogiri-core/pipelines/mini-portile/jobs/ruby-2.7/badge)](https://ci.nokogiri.org/teams/nokogiri-core/pipelines/mini-portile)
-[![Appveyor](https://ci.appveyor.com/api/projects/status/509669xx1qlhqqab/branch/main?svg=true)](https://ci.appveyor.com/project/flavorjones/mini-portile/branch/main)
+[![Continuous Integration](https://github.com/flavorjones/mini_portile/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/flavorjones/mini_portile/actions/workflows/ci.yml)
 [![Tidelift dependencies](https://tidelift.com/badges/package/rubygems/mini_portile2)](https://tidelift.com/subscription/pkg/rubygems-mini.portile2?utm_source=undefined&utm_medium=referral&utm_campaign=readme)
 
 * Documentation: http://www.rubydoc.info/github/flavorjones/mini_portile