[7.6] add 6.8.8 as a version (#53022)

Author: jakelandis

.ci/bwcVersions

@@ -37,6 +37,7 @@ BWC_VERSION:
   - "6.8.5"
   - "6.8.6"
   - "6.8.7"
+  - "6.8.8"
   - "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_5 = new Version(6080599, org.apache.lucene.util.Version.LUCENE_7_7_2);
     public static final Version V_6_8_6 = new Version(6080699, org.apache.lucene.util.Version.LUCENE_7_7_2);
     public static final Version V_6_8_7 = new Version(6080799, org.apache.lucene.util.Version.LUCENE_7_7_2);
+    public static final Version V_6_8_8 = new Version(6080899, org.apache.lucene.util.Version.LUCENE_7_7_2);
     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);