Require Swift 5.6

[fabianfett]

Motivation

We should support the latest Swift releases in the same way Swift NIO does:

From the time of the next non-patch Swift release, the top-level policy of all SwiftNIO projects will be to support the latest non-patch Swift release and the 2 immediately prior non-patch versions. That is, if the next Swift release was Swift 5.6, then we would support Swift 5.6 (the latest), and Swift 5.5 and 5.4 (the two prior versions). The definition of "support" in this context is "fully support", meaning that new features will be implemented across all the supported Swift versions where the language allows, and bugfixes will be applied on all supported versions.

https://forums.swift.org/t/swiftnio-swift-version-support/53232

Changes

• Require Swift 5.6
• Drop code that was needed to support Swift 5.2 and lower