doc: remove extra text from release notes, and keep only changes

Author: romani

net.sf.eclipsecs.doc/src/main/resources/partials/releases/8.21.0/release_notes.html

@@ -1,21 +1,4 @@
 <div>
-    <p>Updates the Checkstyle core to version 8.20.</p>
-    <p>
-        Please note that this version requires
-        <strong>Java 8 as the minimum runtime environment</strong>
-        . This requires
-        you to run Eclipse on a Java 8 JVM in order to use this plugin version.
-    </p>
-    <p>This restriction does not affect the ability to analyze Java 5 or lower level source code.</p>
-    <p>
-        Please read the
-        <a target="_blank" href="http://checkstyle.sourceforge.net/releasenotes.html">Checkstyle
-            release notes
-        </a>
-        for details on recent and potentially breaking changes in Checkstyle.
-        Checkstyle configuration files potentially
-        need to be adapted to this version.
-    </p>
     <h3>Changes</h3>
     <ul>
         <li>