feat: use dependabot to maintain packages

[WilliamBergamin]

Since #996 packages in this project are defined in requirements.txt files, dependabot supports this as a pip package-ecosystem. This PR introduces dependabot as a tool maintain the pip and github-action dependencies of the project

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
• [x] 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.

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.76%. Comparing base (b6de3b3) to head (5906d5a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1037 +/- ## ======================================= Coverage 91.76% 91.76% ======================================= Files 181 181 Lines 6312 6312 ======================================= Hits 5792 5792 Misses 520 520 ```

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