Remove gerrit 3.9

It's EOL.

Change-Id: I563b45074cf4c0bfef3aeb12a509dba83fde8c04

zuul.d/project.yaml

diff --git a/zuul.d/project.yaml b/zuul.d/project.yaml
index 605ebc6..1fdae76 100644
--- a/zuul.d/project.yaml
+++ b/zuul.d/project.yaml
@@ -5,7 +5,6 @@
         - test-gerrit-setup
         - test-gerrit-plugin-build
         - test-gerrit-build-master
-        - test-gerrit-build-stable-3.9
         - test-gerrit-build-stable-3.10
         - test-gerrit-build-stable-3.11
         - test-gerrit-build-stable-3.12

zuul.d/test-jobs.yaml

diff --git a/zuul.d/test-jobs.yaml b/zuul.d/test-jobs.yaml
index 592eed8..581237b 100644
--- a/zuul.d/test-jobs.yaml
+++ b/zuul.d/test-jobs.yaml
@@ -65,11 +65,6 @@
       - playbooks/gerrit/.*
 
 - job:
-    name: test-gerrit-build-stable-3.9
-    parent: test-gerrit-build-master
-    override-checkout: stable-3.9
-
-- job:
     name: test-gerrit-build-stable-3.10
     parent: test-gerrit-build-master
     override-checkout: stable-3.10