README: deprecate Discourse.

Author: MikeMcQuaid

README.md

@@ -1,5 +1,5 @@
 # Homebrew/discussions
 
-Public open-ended discussions. Possible replacement for our [Discourse](https://discourse.brew.sh).
+Public open-ended discussions. Replacement for our, now deprecated, [Discourse](https://discourse.brew.sh).
 
-[:left_speech_bubble: View Discussions](https://github.com/Homebrew/discussions/discussions)
+[💬 View Discussions](https://github.com/Homebrew/discussions/discussions)