ISSUE_TEMPLATE: direct people to GitHub Discussions

Let's trial this and see how it goes.

.github/ISSUE_TEMPLATE/config.yml

 blank_issues_enabled: false
 
 contact_links:
-- name: Get help on Discourse
-  url: https://discourse.brew.sh
-  about: Have a question? Not sure if your issue affects everyone reproducibly? The quickest way to get help is on Homebrew's Discourse!
+- name: Get help in GitHub Discussions
+  url: https://github.com/Homebrew/discussions/discussions
+  about: Have a question? Not sure if your issue affects everyone reproducibly? The quickest way to get help is on Homebrew's GitHub Discussions!
 - name: New issue on Homebrew/homebrew-core
   url: https://github.com/Homebrew/homebrew-core/issues/new/choose
   about: On macOS/Mac OS X? Having a `brew` problem with a `brew install` or `brew upgrade` of a single formula/package? Report it to Homebrew/homebrew-core (the macOS core tap/repository).