Build high-availability plugin for stable-2.{14,15}

There is one fix needed to make the high-availability
working again on stable-2.14 and stable-2.15.
Resume CI validation for those two branches so that
the code can be compiled again and people can easily
upgrade.

Change-Id: Ib5c90c6d193a30545fe7ce719d2665f4afc37bea
diff --git a/jenkins/gerrit-bazel-plugin-high-availability.yaml b/jenkins/gerrit-bazel-plugin-high-availability.yaml
index 8f901e6..76138ed 100644
--- a/jenkins/gerrit-bazel-plugin-high-availability.yaml
+++ b/jenkins/gerrit-bazel-plugin-high-availability.yaml
@@ -12,3 +12,5 @@
             - stable-3.1
             - stable-3.0
             - stable-2.16
+            - stable-2.15
+            - stable-2.14