Fix gitter badge in README (#2118)

README.md

@@ -2,7 +2,7 @@
 
 [![Documentation Status](https://readthedocs.org/projects/esmvalcore/badge/?version=latest)](https://esmvaltool.readthedocs.io/en/latest/?badge=latest)
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3387139.svg)](https://doi.org/10.5281/zenodo.3387139)
-[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/ESMValGroup?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Chat on Matrix](https://matrix.to/img/matrix-badge.svg)](https://matrix.to/#/#ESMValGroup_Lobby:gitter.im)
 [![CircleCI](https://circleci.com/gh/ESMValGroup/ESMValCore/tree/main.svg?style=svg)](https://circleci.com/gh/ESMValGroup/ESMValCore/tree/main)
 [![codecov](https://codecov.io/gh/ESMValGroup/ESMValCore/branch/main/graph/badge.svg?token=wQnDzguwq6)](https://codecov.io/gh/ESMValGroup/ESMValCore)
 [![Codacy Badge](https://app.codacy.com/project/badge/Grade/5d496dea9ef64ec68e448a6df5a65783)](https://www.codacy.com/gh/ESMValGroup/ESMValCore?utm_source=github.com&utm_medium=referral&utm_content=ESMValGroup/ESMValCore&utm_campaign=Badge_Grade)