README: Update Swift version and system requirements

Author: JohnSundell

README.md

@@ -3,7 +3,7 @@
 </p>
 
 <p align="center">
-    <img src="https://img.shields.io/badge/Swift-5.2-orange.svg" />
+    <img src="https://img.shields.io/badge/Swift-5.4-orange.svg" />
     <a href="https://swift.org/package-manager">
         <img src="https://img.shields.io/badge/swiftpm-compatible-brightgreen.svg?style=flat" alt="Swift Package Manager" />
     </a>
@@ -232,7 +232,7 @@ For a real-world example of a Publish plugin, check out the [official Splash plu
 
 ## System requirements
 
-To be able to successfully use Publish, make sure that your system has Swift version 5.2 (or later) installed. If you’re using a Mac, also make sure that `xcode-select` is pointed at an Xcode installation that includes the required version of Swift, and that you’re running macOS Catalina (10.15) or later. 
+To be able to successfully use Publish, make sure that your system has Swift version 5.4 (or later) installed. If you’re using a Mac, also make sure that `xcode-select` is pointed at an Xcode installation that includes the required version of Swift, and that you’re running macOS Big Sur (11.0) or later. 
 
 Please note that Publish **does not** officially support any form of beta software, including beta versions of Xcode and macOS, or unreleased versions of Swift.