Remove the possibility of issue #170 by removing emoji from the boot message only for Windows

[seratch]

This pull request adds a workaround for potential issues on Windows like #170 .

As long as a developer uses UTF-8 as the system's encoding, the issue described at #170 never happens (actually, I was not able to reproduce the issue on my personal Windows machine). That said, if a developer encounters #170, it can be a critical blocker for trying Bolt out.

This pull request simply removes the ⚡️ character from the boot message only when running the app on Windows OS. I like having the emoji in the message but it's more important to enable any developers to try this framework. If we could find a way to safely get the emoji back for Windows, I'm happy to have the additional updates to it.

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.