Change ctor visibility of InitializrProperties.SimpleElement to public

Closes gh-1623

Author: mhalbritter

initializr-metadata/src/main/java/io/spring/initializr/metadata/InitializrProperties.java

@@ -172,7 +172,11 @@ public static final class SimpleElement {
 		 */
 		private String value;
 
-		private SimpleElement(String value) {
+		/**
+		 * Create a new instance with the given value.
+		 * @param value the value
+		 */
+		public SimpleElement(String value) {
 			this.value = value;
 		}