Add announcement secrets to post-release workflow

[tvdeyen]

Adds secrets for Slack, Mastodon, and Bluesky release announcements.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.62%. Comparing base (881a0a0) to head (0c7253c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #244   +/-   ##
=======================================
  Coverage   98.62%   98.62%           
=======================================
  Files          11       11           
  Lines         291      291           
=======================================
  Hits          287      287           
  Misses          4        4           

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🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
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