Bump version after 6.8.17 release

Author: joegallo

.ci/bwcVersions

@@ -83,3 +83,4 @@ BWC_VERSION:
   - "6.8.14"
   - "6.8.15"
   - "6.8.16"
+  - "6.8.17"

buildSrc/version.properties

@@ -1,4 +1,4 @@
-elasticsearch     = 6.8.17
+elasticsearch     = 6.8.18
 lucene            = 7.7.3
 
 # optional dependencies

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

@@ -161,9 +161,9 @@ public class Version implements Comparable<Version>, ToXContentFragment {
     public static final Version V_6_8_15 = new Version(6081599, org.apache.lucene.util.Version.LUCENE_7_7_3);
     public static final Version V_6_8_16 = new Version(6081699, org.apache.lucene.util.Version.LUCENE_7_7_3);
     public static final Version V_6_8_17 = new Version(6081799, org.apache.lucene.util.Version.LUCENE_7_7_3);
+    public static final Version V_6_8_18 = new Version(6081899, org.apache.lucene.util.Version.LUCENE_7_7_3);
 
-    public static final Version CURRENT = V_6_8_17;
-
+    public static final Version CURRENT = V_6_8_18;
 
     private static final ImmutableOpenIntMap<Version> idToVersion;