[Concurrency] Lower availability of addTask* with task name

Author: ktoso

stdlib/public/Concurrency/TaskGroup+addTask.swift.gyb

@@ -31,7 +31,7 @@ import Swift
 %          'ThrowingDiscardingTaskGroup'
 %        ],
 %        [
-%          '@available(SwiftStdlib 6.2, *)',
+%          '@available(SwiftStdlib 5.1, *)',
 %          '@_unavailableInEmbedded', # since TaskExecutor is not available on embedded
 %        ],
 %        ['addTask', 'addTaskUnlessCancelled'],