Make logging optional and update docs

[nesrve]

Hello! 👋 This PR contains a few changes, notably:

• Updating the README.md to use the new initializer instead of the deprecated one.
• Add a renamed to the deprecated initializer.
• Updating the docs to use the simpler initializer.
• Make the logging parameter on the initializers optional, the motivation for this is in cases the user don't or can't do logging, they don't have to construct a logger and bootstrap the logging system with SwiftLogNoOpLogHandler, so this code:

LoggingSystem.bootstrap(SwiftLogNoOpLogHandler.init)
let serviceGroup = ServiceGroup(
    services: [service],
    cancellationSignals: [.sigint],
    logger: .init(label: "")
)

becomes

let serviceGroup = ServiceGroup(
    services: [service],
    cancellationSignals: [.sigint]
)

and it also allows the user to do logging outside the ServiceGroup without necessarily doing logging within it.

If there is anything to improve, let me know! 😃

thanks dimi and gwynne for helping along the way and answering my questions :)

[FranzBusch]

We can't make this change since this an API break. I think at this point there is no nice way to make this optional anymore sadly.

[nesrve]

I see, I've reverted the changes of making it an optional, with only improved docs and readme examples remaining 👍

[dimitribouniol]

An alternative approach to make the logger in the initializers optional could be to chain the log handler from the passed in optional logger to the stored-property logger:

self.logger = Logger(logger.label ?? "ServiceLifecycle", factory: { _ in logger?.handler ?? SwiftLogNoOpLogHandler() })

[FranzBusch]

Thanks for the change. Let's get this landed!