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 

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[zimeg]

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

I look forward to using this in stable code.