feat: upgrade axum to 0.7

[MuZhou233]

Fix #7

• Upgrade axum to 0.7
• Other dependency upgrades are required by axum 0.7
• axum-test-helper doesn't support axum 0.7 right now, switched to axum-test-helpers
• Update README.md

Refer

• https://github.com/tokio-rs/axum/releases/tag/axum-v0.7.0
• https://github.com/hyperium/http-body/blob/master/http-body/CHANGELOG.md

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (master@77f18fb). Click here to learn what that means.

Files	Patch %	Lines
src/middleware.rs	50.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##             master       #9   +/-   ##
=========================================
  Coverage          ?   56.52%           
=========================================
  Files             ?        1           
  Lines             ?       69           
  Branches          ?        0           
=========================================
  Hits              ?       39           
  Misses            ?       30           
  Partials          ?        0           

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