Update Lit & Friends slack links

- previously called Polymer slack

Author: simi

docs/discover/slack.md

@@ -1,5 +1,5 @@
 # Slack
 
-You can also find us on the Polymer Slack in the [#open-wc](https://polymer.slack.com/archives/CE6D9DN05) channel.
+You can also find us on the Lit & Friends Slack in the [#open-wc](https://lit-and-friends.slack.com/archives/CE6D9DN05) channel.
 
-You can join the Polymer Slack by visiting [https://www.polymer-project.org/slack-invite](https://www.polymer-project.org/slack-invite).
+You can join the Lit & Friends Slack by visiting [https://lit.dev/slack-invite](https://lit.dev/slack-invite).