feat: update to edition 2021 and set minimum rust to 1.56

[lucasfernog]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Docs
• New Binding Issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. Issue #___
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[wusyong]

With Minimum Supported Rust Version is finally available, we could remove rust-toolchain file.
Our minimum version would be 1.56 until next edition comes out hopefully.

[lucasfernog]

With Minimum Supported Rust Version is finally available, we could remove rust-toolchain file. Our minimum version would be 1.56 until next edition comes out hopefully.

Yeah that's the plan. I removed the file on this PR.