Update formatting and gate availability for older compilers

Author: phausler

Package.swift

@@ -3,12 +3,20 @@
 import PackageDescription
 import CompilerPluginSupport
 
+let AsyncAlgorithms_v1_0 = "AvailabilityMacro=AsyncAlgorithms 1.0:macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0"
+#if compiler(>=6.0)
+let AsyncAlgorithms_v1_1 =
+  "AvailabilityMacro=AsyncAlgorithms 1.1:macOS 15.0, iOS 18.0, tvOS 18.0, watchOS 11.0, visionOS 2.0"
+#else
+let AsyncAlgorithms_v1_1 = "AvailabilityMacro=AsyncAlgorithms 1.1:macOS 15.0, iOS 18.0, tvOS 18.0, watchOS 11.0"
+#endif
+
 let availabilityMacros: [SwiftSetting] = [
   .enableExperimentalFeature(
-    "AvailabilityMacro=AsyncAlgorithms 1.0:macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0"
+    AsyncAlgorithms_v1_0
   ),
   .enableExperimentalFeature(
-    "AvailabilityMacro=AsyncAlgorithms 1.1:macOS 15.0, iOS 18.0, tvOS 18.0, watchOS 11.0, visionOS 2.0"
+    AsyncAlgorithms_v1_1
   ),
 ]