Update async-http-client to 1.19.0

[iMacHumphries]

What does this PR do?

Updates the min version of async-http-client to 1.19.0 to potentially fix a build issue

[961/962] Compiling Appwrite Client.swift
/usr/local/server/.build/checkouts/sdk-for-swift/Sources/Appwrite/Client.swift:40:83: error: type 'HTTPClient.EventLoopGroupProvider' has no member 'singleton'
    private static var eventLoopGroupProvider = HTTPClient.EventLoopGroupProvider.singleton
                                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~

/usr/src/code/src/Executor/Executor.php:96
#0 /usr/src/code/src/Appwrite/Platform/Workers/Builds.php(408): Executor\Executor-

Test Plan

Check if CI build passes.