Revert incorrect minimum Swift version requirement changes

Author: rhx

README.md

@@ -11,7 +11,7 @@ For up to date (auto-generated) reference documentation, see https://rhx.github.
 
 Support for gtk 4 was added via the `gtk4` branch.
 
-Version 15 of gir2swift provides a Package Manager Plugin.  This requires Swift 5.10 or higher
+Version 15 of gir2swift provides a Package Manager Plugin.  This requires Swift 5.6 or higher
 (older versions can be used via the [swift52](https://github.com/rhx/SwiftGtk/tree/swift52) branch).
 
 ## Usage
@@ -27,7 +27,7 @@ After this, you should be able to `import Gtk` in your sources and use `swift bu
 Alternatively, you can manually download [run-gir2swift.sh](https://github.com/rhx/gir2swift/raw/main/run-gir2swift.sh ) and add `SwiftGtk` as a dependency to your `Package.swift` file, e.g.:
 
 ```Swift
-// swift-tools-version:5.10
+// swift-tools-version:5.6
 
 import PackageDescription
 
@@ -85,7 +85,7 @@ After that, use the (usual) Build and Test buttons to build/test this package.
 
 ## Prerequisites
 
-### Swift 5.10 or higher
+### Swift 5.6 or higher
 
 To build, download Swift from https://swift.org/download/ -- if you are using macOS, make sure you have the command line tools installed as well).  Test that your compiler works using `swift --version`, which should give you something like