Fix visionOS build failure on Swift 6.0.
SyncMutex is declared under #if canImport(Synchronization) && compiler(>=6.1), but AnyMutex referenced it under #if canImport(Synchronization) && supportsVisionOS && compiler(>=6.0). On Swift 6.0 targeting visionOS the declaration was compiled out while the references were not, producing four cannot find 'SyncMutex' in scope errors.
Both guards now match. Side effect: the AnyMutex.syncMutex() / AnyMutex.syncMutex(initialValue:) factories were previously gated behind supportsVisionOS and therefore reachable only on visionOS. They are now available on every platform that provides the Synchronization module, subject to the existing @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *) annotation.