Un-deprecate Task.startOnMainActor (#75544)

Author: Harlan Haskins

stdlib/public/Concurrency/Task.swift

@@ -255,8 +255,6 @@ extension Task where Failure == Error {
     @_spi(MainActorUtilities)
     @MainActor
     @available(SwiftStdlib 5.9, *)
-    @available(*, deprecated, message: "Use Task.init with a main actor isolated closure instead")
-    @available(swift, obsoleted: 6.0, message: "Use Task.init with a main actor isolated closure instead")
     @discardableResult
     public static func startOnMainActor(
         priority: TaskPriority? = nil,
@@ -280,8 +278,6 @@ extension Task where Failure == Never {
     @_spi(MainActorUtilities)
     @MainActor
     @available(SwiftStdlib 5.9, *)
-    @available(*, deprecated, message: "Use Task.init with a main actor isolated closure instead")
-    @available(swift, obsoleted: 6.0, message: "Use Task.init with a main actor isolated closure instead")
     @discardableResult
     public static func startOnMainActor(
         priority: TaskPriority? = nil,