CI: Add 7.13 target to clients CI

Author: Anaethelion

.ci/jobs/elastic+go-elasticsearch+7.13.yml

@@ -0,0 +1,15 @@
+---
+- job:
+    name: elastic+go-elasticsearch+7.13
+    display-name: 'elastic / go-elasticsearch # 7.13'
+    description: Testing the go-elasticsearch 7.13 branch.
+    junit_results: "*-junit.xml"
+    parameters:
+      - string:
+          name: branch_specifier
+          default: refs/heads/7.13
+          description: The Git branch specifier to build
+    triggers:
+      - github
+      - timed: 'H */12 * * *'
+