Motivation

It's good if we compile with the GM Xcode.

Modifications

Guard Duration usability to at least Swift 5.7.1.

Result

NIO compiles on GM Xcode again.

Note that this is technically an API break for those using Xcode 14
Betas. As those users can (and should) update to 14.1 Beta, and we don't
define API stability for Beta xcodes, this is acceptable.