Bump versions for 6.8.20 release

Author: danhermann

.ci/bwcVersions

@@ -86,3 +86,4 @@ BWC_VERSION:
   - "6.8.17"
   - "6.8.18"
   - "6.8.19"
+  - "6.8.20"

buildSrc/version.properties

@@ -1,4 +1,4 @@
-elasticsearch     = 6.8.20
+elasticsearch     = 6.8.21
 lucene            = 7.7.3
 
 # optional dependencies

server/src/main/java/org/elasticsearch/Version.java

@@ -164,8 +164,9 @@ public class Version implements Comparable<Version>, ToXContentFragment {
     public static final Version V_6_8_18 = new Version(6081899, org.apache.lucene.util.Version.LUCENE_7_7_3);
     public static final Version V_6_8_19 = new Version(6081999, org.apache.lucene.util.Version.LUCENE_7_7_3);
     public static final Version V_6_8_20 = new Version(6082099, org.apache.lucene.util.Version.LUCENE_7_7_3);
+    public static final Version V_6_8_21 = new Version(6082199, org.apache.lucene.util.Version.LUCENE_7_7_3);
 
-    public static final Version CURRENT = V_6_8_20;
+    public static final Version CURRENT = V_6_8_21;
 
     private static final ImmutableOpenIntMap<Version> idToVersion;