mark syncShutdownGracefully noasync
Motivation:
The code as-is blocks the calling thread.
Modifications:
* mark `EventLoopGroup.syncShutdownGracefully()` and `NIOThreadPool.syncShutdownGracefully()` noasync on Swift > 5.7
* offer NIOThreadPool.shutdownGracefully()
* add renamed to syncShutdownGracefully()
Rick Newton-Rogers