[Concurrency] Update availability of UnimplementedExecutor.

We need it to be `StdlibDeploymentTarget 6.2` rather than
`SwiftStdlib 6.2`, so that we can update the availability definitions.

rdar://156710569

Author: al45tair

stdlib/public/Concurrency/UnimplementedExecutor.swift

@@ -14,7 +14,7 @@ import Swift
 
 // .. Main Executor ............................................................
 
-@available(SwiftStdlib 6.2, *)
+@available(StdlibDeploymentTarget 6.2, *)
 final class UnimplementedMainExecutor: MainExecutor, @unchecked Sendable {
   public init() {}
 
@@ -45,7 +45,7 @@ final class UnimplementedMainExecutor: MainExecutor, @unchecked Sendable {
 
 // .. Task Executor ............................................................
 
-@available(SwiftStdlib 6.2, *)
+@available(StdlibDeploymentTarget 6.2, *)
 final class UnimplementedTaskExecutor: TaskExecutor, @unchecked Sendable {
   public init() {}