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:

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:

Here is a discussion related with the upgrade:

My Github Page: https://github.com/liweinan

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