chore: remove experiment around say_stream

[WilliamBergamin]

Summary

These changes aim to remove the experiment warning around say stream

Testing

CI should be sufficient

Category

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.31%. Comparing base (dbe1590) to head (96afa5d).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1471      +/-   ##
==========================================
- Coverage   91.32%   91.31%   -0.01%     
==========================================
  Files         229      228       -1     
  Lines        7270     7262       -8     
==========================================
- Hits         6639     6631       -8     
  Misses        631      631              

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[zimeg]

@WilliamBergamin LGTM! When the time is right let's get this merged 🚀 ✨

I look forward to using this in stable code.