Replace JSR305 null-safe annotations by SpotBugs annotations

Reason is that JSR305 annotations cannot be used in a modular environment due to split package issues.

acme4j-it/src/main/java/org/shredzone/acme4j/it/BammBammClient.java

@@ -17,8 +17,6 @@ import java.io.IOException;
 import java.util.Arrays;
 import java.util.Objects;
 
-import javax.annotation.ParametersAreNonnullByDefault;
-
 import org.apache.http.HttpResponse;
 import org.apache.http.HttpStatus;
 import org.apache.http.client.ClientProtocolException;
@@ -33,7 +31,6 @@ import org.shredzone.acme4j.toolbox.JSONBuilder;
 /**
  * The BammBamm client connects to the pebble-challtestsrv.
  */
-@ParametersAreNonnullByDefault
 public class BammBammClient {
     private static final HttpClient CLIENT = HttpClients.createDefault();
 

acme4j-it/src/main/java/org/shredzone/acme4j/it/package-info.java

@@ -0,0 +1,23 @@
+/*
+ * acme4j - Java ACME client
+ *
+ * Copyright (C) 2020 Richard "Shred" Körber
+ *   http://acme4j.shredzone.org
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ */
+
+@ReturnValuesAreNonnullByDefault
+@DefaultAnnotationForParameters(NonNull.class)
+@DefaultAnnotationForFields(NonNull.class)
+package org.shredzone.acme4j.it;
+
+import edu.umd.cs.findbugs.annotations.DefaultAnnotationForFields;
+import edu.umd.cs.findbugs.annotations.DefaultAnnotationForParameters;
+import edu.umd.cs.findbugs.annotations.NonNull;
+import edu.umd.cs.findbugs.annotations.ReturnValuesAreNonnullByDefault;