Replace Slack with Community Forum in issues (#7364)

.github/ISSUE_TEMPLATE/config.yml

 blank_issues_enabled: true
 contact_links:
-  - name: Slack
-    url: https://join.slack.com/t/ultralytics/shared_invite/zt-w29ei8bp-jczz7QYUmDtgo6r6KcMIAg
-    about: Ask on Ultralytics Slack Forum
+  - name: 💬 Forum
+    url: https://community.ultralytics.com/
+    about: Ask on Ultralytics Community Forum
   - name: Stack Overflow
     url: https://stackoverflow.com/search?q=YOLOv5
     about: Ask on Stack Overflow with 'YOLOv5' tag