Gitter link update of Readme.md (#7658)

On last Community Meeting, we have noticed that some active contributors don't know about the Gitter channel. 

This PR replace Gitter badge button by simple web link so it's supposed to be more visible.

README.md

@@ -2,7 +2,7 @@
 
 ![Build Status](https://github.com/espressif/arduino-esp32/workflows/ESP32%20Arduino%20CI/badge.svg) [![Documentation Status](https://readthedocs.com/projects/espressif-arduino-esp32/badge/?version=latest)](https://docs.espressif.com/projects/arduino-esp32/en/latest/?badge=latest)
 
-### Need help or have a question? Join the chat at [![https://gitter.im/espressif/arduino-esp32](https://badges.gitter.im/espressif/arduino-esp32.svg)](https://gitter.im/espressif/arduino-esp32?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) or [open a new Discussion](https://github.com/espressif/arduino-esp32/discussions)
+### Need help or have a question? Join the chat at [Gitter](https://gitter.im/espressif/arduino-esp32) or [open a new Discussion](https://github.com/espressif/arduino-esp32/discussions)
 
 ## Contents