[SE-0323] Implemented in Swift 5.5.x, not 5.6 (#1467)
[SE-0323] Implemented in Swift 5.5.x, not 5.6 (#1467)

* [SE-0323] Implemented in Swift 5.5.2, not 5.6

It looks like the implementation for [SE-0323 Asynchronous Main Semantics](0323-async-main-semantics.md) has been merged into the 5.5 branch (#39607) and [the changelog also lists it under Swift 5.5](https://github.com/apple/swift/blob/main/CHANGELOG.md#swift-55). Update the proposal to reflect this.

* Update proposals/0323-async-main-semantics.md

Co-authored-by: Doug Gregor <20237f4b434bdcccb3b4c7b8643e049a656bb565@apple.com>
abertelrud/swift-evolution,natecook1000/swift-evolution,apple/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution