Use the java.net.http client

With this patch, the old HttpURLConnection client is replaced with
the new java.net.http client that was introduced in Java 11.

acme4j-client/src/main/java/module-info.java

@@ -14,6 +14,7 @@
 
 module org.shredzone.acme4j {
     requires static com.github.spotbugs.annotations;
+    requires java.net.http;
     requires org.jose4j;
     requires org.slf4j;