feat: Specify TLS v1.2 as required minimum (#57)

TLS 1.1 has been deprecated for a long time and has been dropped by most
major providers. The LaunchDarkly APIs haven't accepted anything lower
than TLS 1.2 for a long time.

We are therefore forcing TLS 1.2 as the minimum going forward in this
library.

Author: keelerm84

Source/LDSwiftEventSource.swift

@@ -85,6 +85,14 @@ public class EventSource {
                 let sessionConfig = _urlSessionConfiguration.copy() as! URLSessionConfiguration
                 sessionConfig.httpAdditionalHeaders = ["Accept": "text/event-stream", "Cache-Control": "no-cache"]
                 sessionConfig.timeoutIntervalForRequest = idleTimeout
+
+                #if !os(Linux)
+                if #available(iOS 13, macOS 10.15, tvOS 13, watchOS 6, *) {
+                    sessionConfig.tlsMinimumSupportedProtocolVersion = .TLSv12
+                } else {
+                    sessionConfig.tlsMinimumSupportedProtocol = .tlsProtocol12
+                }
+                #endif
                 return sessionConfig
             }
             set {