Add missing availability annotation to NIOTSConnectionBootstrap (#160)

Motivation:

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

Modifications:

- Add appropriate availability annotation

Result:

Resolves #159

Sources/NIOTransportServices/NIOTSConnectionBootstrap.swift

@@ -263,6 +263,7 @@ public final class NIOTSConnectionBootstrap {
 
 #if swift(>=5.6)
 @available(*, unavailable)
+@available(OSX 10.14, iOS 12.0, tvOS 12.0, watchOS 6.0, *)
 extension NIOTSConnectionBootstrap: Sendable {}
 #endif