Update iOS 13 annotations on library mocks & mock generator

Libraries/ConnectMocks/MockBidirectionalAsyncStream.swift

@@ -24,6 +24,7 @@ import SwiftProtobuf
 ///
 /// To return data over the stream, outputs can be specified using `init(outputs: ...)` or by
 /// subclassing and overriding `results()`.
+@available(iOS 13, *)
 open class MockBidirectionalAsyncStream<
     Input: SwiftProtobuf.Message,
     Output: SwiftProtobuf.Message

Libraries/ConnectMocks/MockClientOnlyAsyncStream.swift

@@ -23,6 +23,7 @@ import SwiftProtobuf
 ///
 /// To return data over the stream, outputs can be specified using `init(outputs: ...)` or by
 /// subclassing and overriding `results()`.
+@available(iOS 13, *)
 open class MockClientOnlyAsyncStream<
     Input: SwiftProtobuf.Message,
     Output: SwiftProtobuf.Message

Libraries/ConnectMocks/MockServerOnlyAsyncStream.swift

@@ -24,6 +24,7 @@ import SwiftProtobuf
 ///
 /// To return data over the stream, outputs can be specified using `init(outputs: ...)` or by
 /// subclassing and overriding `results()`.
+@available(iOS 13, *)
 open class MockServerOnlyAsyncStream<
     Input: SwiftProtobuf.Message,
     Output: SwiftProtobuf.Message

Plugins/ConnectMocksPlugin/ConnectMockGenerator.swift

@@ -57,6 +57,7 @@ final class ConnectMockGenerator: Generator {
         self.printLine("/// class, allowing for mocking RPC calls. Behavior can be customized")
         self.printLine("/// either through the properties on this class or by")
         self.printLine("/// subclassing the class and overriding its methods.")
+        self.printLine("@available(iOS 13, *)")
         self.printLine(
             "\(self.typeVisibility) class \(service.mockName(using: self.namer)): \(protocolName) {"
         )
@@ -82,7 +83,6 @@ final class ConnectMockGenerator: Generator {
                     self.printLine(
                         "/// Mocked for async calls to `\(method.name(using: self.options))()`."
                     )
-                    self.printLine("@available(iOS 13, *)")
                     self.printLine(
                         """
                         \(self.propertyVisibility) var \(method.asyncAwaitMockPropertyName()) = \
@@ -144,7 +144,6 @@ final class ConnectMockGenerator: Generator {
     private func printAsyncAwaitMethodMockImplementation(for method: MethodDescriptor) {
         self.printLine()
 
-        self.printLine("@available(iOS 13, *)")
         self.printLine(
             "\(self.typeVisibility) "
             + method.asyncAwaitSignature(

Tests/ConnectLibraryTests/ConnectCrosstests/AsyncAwaitCrosstests.swift

@@ -27,6 +27,7 @@ private typealias UnimplementedServiceClient = Grpc_Testing_UnimplementedService
 /// Tests are based on https://github.com/bufbuild/connect-crosstest
 ///
 /// Tests are written using async/await APIs.
+@available(iOS 13, *)
 final class AsyncAwaitCrosstests: XCTestCase {
     private func executeTestWithClients(
         function: Selector = #function,

Tests/ConnectLibraryTests/ConnectMocksTests/ConnectMocksTests.swift

@@ -18,6 +18,7 @@ import SwiftProtobuf
 import XCTest
 
 /// Test suite that validates the behavior of generated mock classes.
+@available(iOS 13, *)
 final class ConnectMocksTests: XCTestCase {
     // MARK: - Unary