[core] cleanup workaround using NIOLockedValueBox instead of Mutex

[sebsto]

Due to
swiftlang/swift#80036
swiftlang/swift#78048

we use NIOLockedValueBox instead of Mutex in LambdaRuntime.swift

This issue to is keep track of that and to not forget remove NIOLockedValueBox when 6.0.x will fix this or when we will not support 6.0 anymore (at the release of 6.3 in 2026)

[sebsto]

When #508 will be merged, the usage of NIOLockedValueBox is now in Utils.swift

[sebsto]

Fixed in swiftlang/swift#76303
Will land in 6.3, probably 6.2.1
But there will be no more releases of 6.0 and 6.1
As we also support 6.0 and 6.1, we will need to wait for 6.4 to cleanup our code (when we will support 6.2, 6.3, and 6.4)