Add new Slack sign-up link (envoyproxy#23436)

Signed-off-by: Matt Klein <[email protected]>

Author: mattklein123

README.md

@@ -47,9 +47,7 @@ to find out more about the origin story and design philosophy of Envoy
 * [envoy-maintainers](https://groups.google.com/forum/#!forum/envoy-maintainers): Use this list
   to reach all core Envoy maintainers.
 * [Twitter](https://twitter.com/EnvoyProxy/): Follow along on Twitter!
-* [Slack](https://envoyproxy.slack.com/): Slack, to get invited go [here](https://envoyproxy.io/slack).
-  We have the IRC/XMPP gateways enabled if you prefer either of those. Once an account is created,
-  connection instructions for IRC/XMPP can be found [here](https://envoyproxy.slack.com/account/gateways).
+* [Slack](https://envoyproxy.slack.com/): Slack, to get invited go [here](https://communityinviter.com/apps/envoyproxy/envoy).
   * NOTE: Response to user questions is best effort on Slack. For a "guaranteed" response please email
     envoy-users@ per the guidance in the following linked thread.