Update UI label to indicate it's Java 15+ only

Author: andreweacott

idea-plugin/src/main/java/com/palantir/javaformat/intellij/PalantirJavaFormatConfigurable.form

@@ -21,7 +21,7 @@
           <grid row="1" column="0" row-span="1" col-span="2" vsize-policy="0" hsize-policy="3" anchor="8" fill="0" indent="0" use-parent-layout="false"/>
         </constraints>
         <properties>
-          <text value="Skip reflowing long strings"/>
+          <text value="Skip reflowing long strings (Java 15+)"/>
         </properties>
       </component>
       <component id="c93e1" class="javax.swing.JLabel">