Remove 2020-resolver

[WilliamBergamin]

The pip 22.3 release notes mentions that #11493 removed the --use-feature=2020-resolver opt-in flag. This PR removes the instructions that asked contributors to add this flag.

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 #745 (1cf5fbf) into main (1a3ff44) will not change coverage.
The diff coverage is n/a.

❗ Current head 1cf5fbf differs from pull request most recent head c4d7d65. Consider uploading reports for the commit c4d7d65 to get more accurate results

@@           Coverage Diff           @@
##             main     #745   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         172      172           
  Lines        5875     5875           
=======================================
  Hits         5409     5409           
  Misses        466      466           

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[seratch]

Thank you!