Remove references to discourse.joinmastodon.org (#17797)

Remove broken CODEOWNERS file

Add sponsor.joinmastodon.org to FUNDING.yml

.github/ISSUE_TEMPLATE/config.yml

@@ -1,5 +1,8 @@
 blank_issues_enabled: false
 contact_links:
-  - name: Mastodon Meta Discussion Board
-    url: https://discourse.joinmastodon.org/
+  - name: GitHub Discussions
+    url: https://github.com/mastodon/mastodon/discussions
     about: Please ask and answer questions here.
+  - name: Bug Bounty Program
+    url: https://app.intigriti.com/programs/mastodon/mastodonio/detail
+    about: Please report security vulnerabilities here.