Update Discord invite links

CONTRIBUTING.rst

@@ -13,7 +13,7 @@ the following resources for questions about using Flask or issues with
 your own code:
 
 -   The ``#get-help`` channel on our Discord chat:
-    https://discord.gg/t6rrQZH
+    https://discord.gg/pallets
 -   The mailing list flask@python.org for long term discussion or larger
     issues.
 -   Ask on `Stack Overflow`_. Search with Google first using:

README.rst

@@ -72,7 +72,7 @@ Links
 * Code: https://github.com/pallets/flask
 * Issue tracker: https://github.com/pallets/flask/issues
 * Test status: https://dev.azure.com/pallets/flask/_build
-* Official chat: https://discord.gg/t6rrQZH
+* Official chat: https://discord.gg/pallets
 
 .. _WSGI: https://wsgi.readthedocs.io
 .. _Werkzeug: https://www.palletsprojects.com/p/werkzeug/

docs/extensiondev.rst

@@ -332,5 +332,5 @@ ecosystem remain consistent and compatible.
 .. _PyPI: https://pypi.org/search/?c=Framework+%3A%3A+Flask
 .. _OAuth extension: https://pythonhosted.org/Flask-OAuth/
 .. _mailinglist: https://mail.python.org/mailman/listinfo/flask
-.. _Discord server: https://discord.gg/t6rrQZH
+.. _Discord server: https://discord.gg/pallets
 .. _Official Pallets Themes: https://pypi.org/project/Pallets-Sphinx-Themes/