Bump version after 6.8.17 release

Author: joegallo

.ci/bwcVersions

@@ -47,6 +47,7 @@ BWC_VERSION:
   - "6.8.15"
   - "6.8.16"
   - "6.8.17"
+  - "6.8.18"
   - "7.0.0"
   - "7.0.1"
   - "7.1.0"

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

@@ -112,6 +112,7 @@ public class Version implements Comparable<Version>, ToXContentFragment {
     public static final Version V_6_8_15 = new Version(6081599, LUCENE_7_7_3);
     public static final Version V_6_8_16 = new Version(6081699, LUCENE_7_7_3);
     public static final Version V_6_8_17 = new Version(6081799, LUCENE_7_7_3);
+    public static final Version V_6_8_18 = new Version(6081899, LUCENE_7_7_3);
     public static final Version V_7_0_0 = new Version(7000099, org.apache.lucene.util.Version.LUCENE_8_0_0);
     public static final Version V_7_0_1 = new Version(7000199, org.apache.lucene.util.Version.LUCENE_8_0_0);
     public static final Version V_7_1_0 = new Version(7010099, org.apache.lucene.util.Version.LUCENE_8_0_0);