[stdlib] Define placeholder availability for SwiftStdlib 6.1

Author: lorentey

utils/availability-macros.def

@@ -36,6 +36,7 @@ SwiftStdlib 5.8:macOS 13.3, iOS 16.4, watchOS 9.4, tvOS 16.4
 SwiftStdlib 5.9:macOS 14.0, iOS 17.0, watchOS 10.0, tvOS 17.0
 SwiftStdlib 5.10:macOS 14.4, iOS 17.4, watchOS 10.4, tvOS 17.4, visionOS 1.1
 SwiftStdlib 6.0:macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0
+SwiftStdlib 6.1:macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, visionOS 9999
 # TODO: Also update ASTContext::getSwift510Availability when needed
 # TODO: Also update ASTContext::getSwift60Availability when needed