Change SwiftSyntax dependency branch to release/5.9

Author: ahoppen

Package.swift

@@ -221,7 +221,7 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
     ),
     .package(
       url: "https://github.com/apple/swift-syntax.git",
-      branch: "main"
+      branch: "release/5.9"
     ),
     .package(
       url: "https://github.com/apple/swift-tools-support-core.git",