docs: replace gitter links with discord (#230)

Author: Selflocking

CONTRIBUTING.md

@@ -8,7 +8,7 @@ This project adheres to the [Contributor Covenant 1.2.](https://www.contributor-
 
 * We do our best to have un [up to date documentation](https://casbin.org/docs/en/overview)
 * [Stack Overflow](https://stackoverflow.com) is the best place to start if you have a question. Please use the [casbin tag](https://stackoverflow.com/tags/casbin/info) we are actively monitoring. We encourage you to use Stack Overflow specially for Modeling Access Control Problems, in order to build a shared knowledge base.
-* You can also join our [Gitter community](https://gitter.im/casbin/Lobby).
+* You can also join our [Discord](https://discord.gg/S5UjpzGZjN).
 
 
 ## Reporting issues

README.md

@@ -4,7 +4,7 @@ Casbin-CPP
 [![CI](https://github.com/casbin/casbin-cpp/actions/workflows/ci.yml/badge.svg)](https://github.com/casbin/casbin-cpp/actions/workflows/ci.yml)
 [![GitHub release (latest by date)](https://img.shields.io/github/v/release/casbin/casbin-cpp)](https://github.com/casbin/casbin-cpp/releases/latest)
 [![semantic-release](https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg)](https://github.com/semantic-release/semantic-release)
-[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/casbin/lobby)
+[![Discord](https://img.shields.io/discord/1022748306096537660?logo=discord&label=discord&color=5865F2)](https://discord.gg/S5UjpzGZjN)
 
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