Drop Swift versions earlier than 5.8 (apple#299)

* Drop Swift versions earlier than 5.8

* Delete build_podspec.sh

* Update check_no_api_breakages script

Author: gjcairo

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.8
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the Swift Logging API open source project

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@@ -10,7 +10,7 @@ If you have a server-side Swift application, or maybe a cross-platform (for exam
 
 #### Adding the dependency
 
-`SwiftLog` is designed for Swift 5. To depend on the logging API package, you need to declare your dependency in your `Package.swift`:
+`SwiftLog` is designed for Swift 5.8 and later. To depend on the logging API package, you need to declare your dependency in your `Package.swift`:
 
 ```swift
 .package(url: "https://github.com/apple/swift-log.git", from: "1.0.0"),
@@ -19,10 +19,6 @@ If you have a server-side Swift application, or maybe a cross-platform (for exam
 and to your application/library target, add `"Logging"` to your `dependencies`, e.g. like this:
 
 ```swift
-// Target syntax for Swift up to version 5.1
-.target(name: "BestExampleApp", dependencies: ["Logging"]),
-
-// Target for Swift 5.2
 .target(name: "BestExampleApp", dependencies: [
     .product(name: "Logging", package: "swift-log")
 ],

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@@ -212,8 +212,4 @@ extension LogHandler {
 
 // MARK: - Sendable support helpers
 
-#if compiler(>=5.6)
 @preconcurrency public protocol _SwiftLogSendableLogHandler: Sendable {}
-#else
-public protocol _SwiftLogSendableLogHandler {}
-#endif