feat: rename NOTIFICATIONS_SLACK_APP_TOKEN to NOTIFICATIONS_SLACK_BOT_TOKEN and drop legacy parameter as_user

[fpintoppb]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.04%. Comparing base (215cc88) to head (12d7ceb).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
- Coverage   81.11%   81.04%   -0.08%     
==========================================
  Files          14       14              
  Lines         519      517       -2     
==========================================
- Hits          421      419       -2     
  Misses         98       98              

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