Update our locks to use Synchronization.Mutex or os_unfair_lock

[asdf-bro]

Now that we require Swift 6.0, update LockStorage to use Synchronization.Mutex or os_unfair_lock

Motivation:

Simplify and optimize NIOLock and NIOLockedValueBox on every platform

Modifications:

• Moved all pthread_mutex_t and SRWLOCK code into lock.swift, to support our condition variable type
• Rebuild LockStorage around Synchronization.Mutex on most platforms
• On Darwin, rebuild around os_unfair_lock instead to support back deployment
• On Darwin, the pointer technique is copied from ManagedAtomic from swift-atomics

Result:

• Much simpler code
• Better or equal performance

[fabianfett]

Mutex has platform availability guards:

@available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
@frozen public struct Mutex<Value> : ~Copyable where Value : ~Copyable {

How do we want to adopt that without increasing our platform requirements?

[Lukasa]

Only adopt it on non-Apple platforms.