Upgrade the `axum-playground` project to use `axum 0.6`

Recently I have upgraded my GitHub - liweinan/axum-playground: My Axum Playground to use axum 0.6, which is a major version upgrade that breaks backward compatibility. Here is the release note of axum 0.6:

• axum - v0.6.0

The release note contains description to the changes, and how to fix the non backward compatibility issues.

Here are the changes in axum-playground upgrade:

• liweinan/axum - upgrade axum to 0.6

Here is a discussion related with the upgrade:

• After upgrading to axum 0.6, all my post methods are broken · Discussion #1755 · tokio-rs/axum · GitHub

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