Update Swift version references in docs

README.md

@@ -78,7 +78,7 @@ The following table maps existing package releases to their minimum required Swi
 | Package version         | Swift version | Xcode release |
 | ----------------------- | ------------- | ------------- |
 | swift-collections 1.0.x | >= Swift 5.3  | >= Xcode 12   |
-| swift-collections 1.1.x | >= Swift 5.6  | >= Xcode 13.3 |
+| swift-collections 1.1.x | >= Swift 5.7  | >= Xcode 13.3 |
 
 (Note: the package has no minimum deployment target, so while it does require clients to use a recent Swift toolchain to build it, the code itself is able to run on any OS release that supports running Swift code.)
 
@@ -88,7 +88,7 @@ The following table maps existing package releases to their minimum required Swi
 To use this package in a SwiftPM project, you need to set it up as a package dependency:
 
 ```swift
-// swift-tools-version:5.7
+// swift-tools-version:5.9
 import PackageDescription
 
 let package = Package(

Sources/HashTreeCollections/TreeDictionary/TreeDictionary.swift

@@ -50,7 +50,7 @@
 /// The tree structure also eliminates the need to reserve capacity in advance:
 /// `TreeDictionary` creates, destroys and resizes individual nodes as
 /// needed, always consuming just enough memory to store its contents. As of
-/// Swift 5.7, the standard collection types never shrink their storage, so
+/// Swift 5.9, the standard collection types never shrink their storage, so
 /// temporary storage spikes can linger as unused but still allocated memory
 /// long after the collection has shrunk back to its usual size.
 ///

Sources/HashTreeCollections/TreeSet/TreeSet.swift

@@ -45,7 +45,7 @@
 ///
 /// The tree structure also eliminates the need to reserve capacity in advance:
 /// `TreeSet` creates, destroys and resizes individual nodes as needed,
-/// always consuming just enough memory to store its contents. As of Swift 5.7,
+/// always consuming just enough memory to store its contents. As of Swift 5.9,
 /// the standard collection types never shrink their storage, so temporary
 /// storage spikes can linger as unused but still allocated memory long after
 /// the collection has shrunk back to its usual size.