[DOCS] Add 7.17.21 release notes (#2224)

Author: jbaiera

docs/src/reference/asciidoc/appendix/release-notes/7.17.21.adoc

@@ -0,0 +1,5 @@
+[[eshadoop-7.17.21]]
+== Elasticsearch for Apache Hadoop version 7.17.21
+
+ES-Hadoop 7.17.21 is a version compatibility release, tested specifically against
+Elasticsearch 7.17.21.

docs/src/reference/asciidoc/appendix/release.adoc

@@ -9,6 +9,7 @@ This section summarizes the changes in each release.
 [[release-notes-7]]
 ===== 7.x
 
+* <<eshadoop-7.17.21>>
 * <<eshadoop-7.17.20>>
 * <<eshadoop-7.17.19>>
 * <<eshadoop-7.17.18>>
@@ -136,6 +137,7 @@ Elasticsearch 5.3.1.
 
 ////////////////////////
 
+include::release-notes/7.17.21.adoc[]
 include::release-notes/7.17.20.adoc[]
 include::release-notes/7.17.19.adoc[]
 include::release-notes/7.17.18.adoc[]