Increment SwiftPM tools version to 5.5 and minimum deployment target to 10.15.4 on macOS (#3875)

* Increase SwiftPM's minimum deployment target from 10.15 to 10.15.4

* Increase SwiftPM's tools version to 5.5

Motivation: Allow use of 5.5 features such as automatic Codable synthesis, and use of Swift-friendlier FileHandle APIs that require 10.15.4 (so we can minimize use of stdio).

Author: abertelrud

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.4
+// swift-tools-version:5.5
 
 /*
  This source file is part of the Swift.org open source project
@@ -63,7 +63,7 @@ if useSwiftCryptoV2 {
 let package = Package(
     name: "SwiftPM",
     platforms: [
-        .macOS(.v10_15),
+        .macOS("10.15.4"),
         .iOS(.v13)
     ],
     products: