HTML5ify Javadoc for xpack core (#30277)

xpack core contains a fork of `Cron` from quartz who's javadoc has a
`<table>` with non-html5 compatible stuff. This html5ifies the table and
switches the `:x-pack:plugin:core` project to building javadoc with
HTML5.

Author: nik9000

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -561,7 +561,6 @@ class BuildPlugin implements Plugin<Project> {
              */
             List html4Projects = [
                 ':server',
-                ':x-pack:plugin:core',
             ]
             if (false == html4Projects.contains(project.path)) {
                 javadoc.options.addBooleanOption('html5', true)

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/scheduler/Cron.java

@@ -6,7 +6,6 @@
 package org.elasticsearch.xpack.core.scheduler;
 
 import org.elasticsearch.ElasticsearchParseException;
-import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.common.xcontent.ToXContentFragment;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.joda.time.DateTimeZone;
@@ -29,8 +28,7 @@
 
 
 /**
- *
- * THIS CLASS IS A COPY OF
+ * THIS CLASS IS A FORK OF
  * <a href="https://fisheye.terracotta.org/browse/Quartz/trunk/quartz-core/src/main/java/org/quartz/CronExpression.java?r=2426">
  *     {@code CronExpression}</a>
  * FROM THE <a href="http://quartz-scheduler.org/">QUARTZ</a> PROJECT
@@ -44,63 +42,63 @@
  * Cron expressions are comprised of 6 required fields and one optional field
  * separated by white space. The fields respectively are described as follows:
  *
- * <table cellspacing="8">
+ * <table style="border-collapse: separate; border-spacing: 8px;">
  * <caption>Fields in cron expressions</caption>
  * <tr>
- * <th align="left">Field Name</th>
- * <th align="left">&nbsp;</th>
- * <th align="left">Allowed Values</th>
- * <th align="left">&nbsp;</th>
- * <th align="left">Allowed Special Characters</th>
+ * <th>Field Name</th>
+ * <th>&nbsp;</th>
+ * <th>Allowed Values</th>
+ * <th>&nbsp;</th>
+ * <th>Allowed Special Characters</th>
  * </tr>
  * <tr>
- * <td align="left"><code>Seconds</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>0-59</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>, - * /</code></td>
+ * <td><code>Seconds</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>0-59</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>, - * /</code></td>
  * </tr>
  * <tr>
- * <td align="left"><code>Minutes</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>0-59</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>, - * /</code></td>
+ * <td><code>Minutes</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>0-59</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>, - * /</code></td>
  * </tr>
  * <tr>
- * <td align="left"><code>Hours</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>0-23</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>, - * /</code></td>
+ * <td><code>Hours</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>0-23</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>, - * /</code></td>
  * </tr>
  * <tr>
- * <td align="left"><code>Day-of-month</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>1-31</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>, - * ? / L W</code></td>
+ * <td><code>Day-of-month</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>1-31</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>, - * ? / L W</code></td>
  * </tr>
  * <tr>
- * <td align="left"><code>Month</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>0-11 or JAN-DEC</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>, - * /</code></td>
+ * <td><code>Month</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>0-11 or JAN-DEC</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>, - * /</code></td>
  * </tr>
  * <tr>
- * <td align="left"><code>Day-of-Week</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>1-7 or SUN-SAT</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>, - * ? / L #</code></td>
+ * <td><code>Day-of-Week</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>1-7 or SUN-SAT</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>, - * ? / L #</code></td>
  * </tr>
  * <tr>
- * <td align="left"><code>Year (Optional)</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>empty, 1970-2199</code></td>
- * <td align="left">&nbsp;</td>
- * <td align="left"><code>, - * /</code></td>
+ * <td><code>Year (Optional)</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>empty, 1970-2199</code></td>
+ * <td>&nbsp;</td>
+ * <td><code>, - * /</code></td>
  * </tr>
  * </table>
  * <P>