Add missing availability annotation to NIOTSConnectionBootstrap

[glbrntt]

Motivation:

• NIOTSConnectionBootstrap was extended to be marked as non-Sendable, this extenion missed the availability annotations.

Modifications:

• Add appropriate availability annotation

Result:

Resolves #159

[Skoti]

I think NIOFilterEmptyWritesHandler also needs to be fixed:

swift-nio-transport-services/Sources/NIOTransportServices/NIOFilterEmptyWritesHandler.swift

Lines 88 to 94 in b39e53a

	#if swift(>=5.6)
	@available(*, unavailable)
	extension NIOFilterEmptyWritesHandler: Sendable {}
	#endif
	// Connection state management
	extension NIOFilterEmptyWritesHandler {

[glbrntt]

I think NIOFilterEmptyWritesHandler also needs to be fixed:

swift-nio-transport-services/Sources/NIOTransportServices/NIOFilterEmptyWritesHandler.swift

Lines 88 to 94 in b39e53a

	#if swift(>=5.6)
	@available(*, unavailable)
	extension NIOFilterEmptyWritesHandler: Sendable {}
	#endif
	// Connection state management
	extension NIOFilterEmptyWritesHandler {

NIOFilterEmptyWritesHandler doesn't have availability requirements:

swift-nio-transport-services/Sources/NIOTransportServices/NIOFilterEmptyWritesHandler.swift

Line 21 in b39e53a

public final class NIOFilterEmptyWritesHandler: ChannelDuplexHandler {