docs: Remove Gitter (#2678)

Remove Gitter (which is now hosted on Matrix) since it's been abandoned for way too long

README.md

@@ -1,6 +1,6 @@
 ![act-logo](https://raw.githubusercontent.com/wiki/nektos/act/img/logo-150.png)
 
-# Overview [![push](https://github.com/nektos/act/workflows/push/badge.svg?branch=master&event=push)](https://github.com/nektos/act/actions) [![Join the chat at https://gitter.im/nektos/act](https://badges.gitter.im/nektos/act.svg)](https://gitter.im/nektos/act?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![Go Report Card](https://goreportcard.com/badge/github.com/nektos/act)](https://goreportcard.com/report/github.com/nektos/act) [![awesome-runners](https://img.shields.io/badge/listed%20on-awesome--runners-blue.svg)](https://github.com/jonico/awesome-runners)
+# Overview [![push](https://github.com/nektos/act/workflows/push/badge.svg?branch=master&event=push)](https://github.com/nektos/act/actions) [![Go Report Card](https://goreportcard.com/badge/github.com/nektos/act)](https://goreportcard.com/report/github.com/nektos/act) [![awesome-runners](https://img.shields.io/badge/listed%20on-awesome--runners-blue.svg)](https://github.com/jonico/awesome-runners)
 
 > "Think globally, `act` locally"
 
@@ -27,7 +27,7 @@ Please look at the [act user guide](https://nektosact.com) for more documentatio
 
 # Support
 
-Need help? Ask on [Gitter](https://gitter.im/nektos/act)!
+Need help? Ask in [discussions](https://github.com/nektos/act/discussions)!
 
 # Contributing