chore: remove 3.6 CI support

[WilliamBergamin]

Summary

The Ubuntu 20.04 Actions runner image will begin deprecation on 2025-02-01 and will be fully unsupported by 2025-04-15 (actions/runner-images#11101). The next available image is Ubuntu 22.04, but it does not support Python 3.6.

Error: The version '3.6' with architecture 'x64' was not found for Ubuntu 22.04.

The project is in the process of dropping support for python 3.6 until this is done the team will need to manually validate compatibility with python 3.6

Steps:

1. pyenv shell 3.6.15
2. python --version -> Python 3.6.15
3. python -m venv env_3.6.15
4. source env_3.6.15/bin/activate
5. scripts/install_all_and_run_tests.sh
6. No tests fail

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 90.96%. Comparing base (5f32fac) to head (5b977e5).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1290   +/-   ##
=======================================
  Coverage   90.96%   90.96%           
=======================================
  Files         222      222           
  Lines        7501     7501           
=======================================
  Hits         6823     6823           
  Misses        678      678           

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[WilliamBergamin]

@ashleymcm thank you for the swift review 💯