chore(deps): update dependency swift-server/async-http-client to from: "1.21.0"

[renovate[bot]]

This PR contains the following updates:

Package	Update	Change
swift-server/async-http-client	minor	from: "1.20.1" -> from: "1.21.0"

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Release Notes

swift-server/async-http-client (swift-server/async-http-client)

### [`v1.21.0`](https://togithub.com/swift-server/async-http-client/releases/tag/1.21.0): AsyncHTTPClient 1.21.0 [Compare Source](https://togithub.com/swift-server/async-http-client/compare/1.20.1...1.21.0) ##### SemVer Minor - Introduce `HTTPClient.shared` a globally shared singleton ([#​705](https://togithub.com/swift-server/async-http-client/issues/705)) - Raise minimum Swift version to 5.8 ([#​729](https://togithub.com/swift-server/async-http-client/issues/729)) ##### SemVer Patch - Add support for Musl ([#​726](https://togithub.com/swift-server/async-http-client/issues/726)) - Fix write timeout not being initialised ([#​730](https://togithub.com/swift-server/async-http-client/issues/730)) ##### Other Changes - Renew certificates in tests ([#​731](https://togithub.com/swift-server/async-http-client/issues/731)) - Move availability guard to correct test ([#​734](https://togithub.com/swift-server/async-http-client/issues/734))

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