ci: update slack action vm version

Switch Slack action to use `ubuntu-latest` like our other actions.

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Co-authored-by: Jordan Woods <[email protected]>

Author: jorwoods

.github/workflows/slack.yml

@@ -5,7 +5,7 @@ on: [push, pull_request, issues]
 jobs:
   slack-notifications:
     continue-on-error: true
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     name: Sends a message to Slack when a push, a pull request or an issue is made
     steps:
       - name: Send message to Slack API