Add 7.9 CI job

Author: russcam

.ci/jobs/elastic+elasticsearch-rs+7.9.yml

@@ -0,0 +1,14 @@
+---
+- job:
+    name: elastic+elasticsearch-rs+7.9
+    display-name: 'elastic / elasticsearch-rs # 7.9'
+    description: Testing the elasticsearch-rs 7.9 branch.
+    junit_results: "*-junit.xml"
+    parameters:
+      - string:
+          name: branch_specifier
+          default: refs/heads/7.9
+          description: The Git branch specifier to build
+    triggers:
+      - github
+      - timed: '@daily'