Strict concurrency for TCP examples (#3103)

Motivation:

Two more examples with no changes required.

Modifications:

Lock in strict concurrency for NIOTCPEchoClient and NIOTCPEchoServer.

Result:

Even more stricter concurrencier.

Author: Lukasa

Package.swift

@@ -275,15 +275,17 @@ let package = Package(
                 "NIOPosix",
                 "NIOCore",
             ],
-            exclude: ["README.md"]
+            exclude: ["README.md"],
+            swiftSettings: strictConcurrencySettings
         ),
         .executableTarget(
             name: "NIOTCPEchoClient",
             dependencies: [
                 "NIOPosix",
                 "NIOCore",
             ],
-            exclude: ["README.md"]
+            exclude: ["README.md"],
+            swiftSettings: strictConcurrencySettings
         ),
         .executableTarget(
             name: "NIOEchoServer",