Move platform requirements to availability annotations

Adding or raising the deployment platforms in the package manifest is a
SemVer major breaking change as consumers must also add or raise their
deployment platforms. This is a known limitation of SwiftPM.

Unforunately this means that it's very difficult for non-leaf packages
to adopt packages which declare their platforms in the manifest. Doing
so puts the brakes on adoption and ecosystem growth. For 'core' packages
like this one availability constraints should be expressed on
declarations rather than in the manifest.

This patch adds equivalent availability annotations to declarations
across the package and removes platforms from the package manifest.

Author: glbrntt

Package.swift

@@ -4,12 +4,6 @@ import PackageDescription
 
 let package = Package(
   name: "swift-async-algorithms",
-  platforms: [
-    .macOS("10.15"),
-    .iOS("13.0"),
-    .tvOS("13.0"),
-    .watchOS("6.0")
-  ],
   products: [
     .library(name: "AsyncAlgorithms", targets: ["AsyncAlgorithms"]),
   ],

Package@swift-5.7.swift

@@ -4,12 +4,6 @@ import PackageDescription
 
 let package = Package(
   name: "swift-async-algorithms",
-  platforms: [
-    .macOS("10.15"),
-    .iOS("13.0"),
-    .tvOS("13.0"),
-    .watchOS("6.0")
-  ],
   products: [
     .library(name: "AsyncAlgorithms", targets: ["AsyncAlgorithms"]),
     .library(name: "AsyncSequenceValidation", targets: ["AsyncSequenceValidation"]),