[OVN] Pin OVS version for OVN v21.06.0

According to [1], the needed version for OVN v21.06.0 is
a4b04276ab5934d087669ff2d191a23931335c87

[1]https://github.com/ovn-org/ovn/commit/4b0a02635fde3d356851dddf12936d9ff2fcc874

Closes-Bug: #1967472
Change-Id: I103f3a63b1d95f79b9e72f1bad197a39a4713da3
diff --git a/zuul.d/master_jobs.yaml b/zuul.d/master_jobs.yaml
index 5085afd..d19aae3 100644
--- a/zuul.d/master_jobs.yaml
+++ b/zuul.d/master_jobs.yaml
@@ -488,7 +488,7 @@
         # is included in an ovn released version
         OVN_BUILD_FROM_SOURCE: True
         OVN_BRANCH: "v21.06.0"
-        OVS_BRANCH: "branch-2.15"
+        OVS_BRANCH: "a4b04276ab5934d087669ff2d191a23931335c87"
         OVS_SYSCONFDIR: "/usr/local/etc/openvswitch"
       devstack_services:
         br-ex-tcpdump: true