Upgrade the minimum slack-sdk version to 3.4

[seratch]

As we have improvements for proxy supports and stability of built-in SocketModeClient in slack-sdk v3.4, we set the minimum required version to the minor version. Also, we stop limiting the usage of newer minor versions since this version.

Category (place an x in each of the [ ])

• 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 (place an x in each [ ])

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

Merging #243 (50dbadf) into main (3bde6df) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   91.36%   91.36%           
=======================================
  Files         160      160           
  Lines        4967     4967           
=======================================
  Hits         4538     4538           
  Misses        429      429           

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