[Concurrency][Tests] Bump availability in a couple of tests.

Since we've bumped availability elsewhere, we need to bump these
tests also.

Author: al45tair

test/Concurrency/Runtime/isolated_conformance.swift

@@ -203,7 +203,7 @@ await Task.detached { @SomeGlobalActor in
 // CHECK: Testing a separate task off the main actor
 print("Testing a separate task off the main actor")
 await Task.detached {
-  if #available(SwiftStdlib 6.2, *) {
+  if #available(SwiftStdlib 6.3, *) {
     // Skip tests on platforms that use the same executor for the main
     // actor and the global concurrent executor.
     guard Task.defaultExecutor !== MainActor.executor else { return }

test/Concurrency/Runtime/sleep_executor.swift

@@ -15,14 +15,14 @@
 import Dispatch
 import StdlibUnittest
 
-@available(SwiftStdlib 6.2, *)
+@available(SwiftStdlib 6.3, *)
 actor MyActor {
   public func doSleep() async {
     try! await Task.sleep(for: .seconds(0.1))
   }
 }
 
-@available(SwiftStdlib 6.2, *)
+@available(SwiftStdlib 6.3, *)
 final class TestExecutor: TaskExecutor, SchedulingExecutor, @unchecked Sendable {
   var asScheduling: SchedulingExecutor? {
     return self
@@ -58,7 +58,7 @@ final class TestExecutor: TaskExecutor, SchedulingExecutor, @unchecked Sendable
   }
 }
 
-@available(SwiftStdlib 6.2, *)
+@available(SwiftStdlib 6.3, *)
 @main struct Main {
   static func main() async {
     let tests = TestSuite("sleep_executor")