Update Docker debian variant for Jenkins master

The migration to buster was missing the update of
the config.xml used for testing of the Jenkins setup.

Align the debian variant to buster.

Change-Id: Ie2be75cbbc91ac443adfdba869e332b178985923
diff --git a/jenkins-docker/master/config.xml b/jenkins-docker/master/config.xml
index 0304610..0990b0e 100644
--- a/jenkins-docker/master/config.xml
+++ b/jenkins-docker/master/config.xml
@@ -58,7 +58,7 @@
           </retentionStrategy>
           <numExecutors>1</numExecutors>
           <dockerTemplateBase>
-            <image>gerritforge/gerrit-ci-slave-bazel-docker:debian-stretch-80.0.3987.149</image>
+            <image>gerritforge/gerrit-ci-slave-bazel-docker:debian-buster-80.0.3987.149</image>
             <dockerCommand></dockerCommand>
             <lxcConfString></lxcConfString>
             <hostname></hostname>
@@ -105,7 +105,7 @@
           </retentionStrategy>
           <numExecutors>1</numExecutors>
           <dockerTemplateBase>
-            <image>gerritforge/gerrit-ci-slave-bazel-docker:debian-stretch-69.0.3497.92</image>
+            <image>gerritforge/gerrit-ci-slave-bazel-docker:debian-buster-69.0.3497.92</image>
             <dockerCommand></dockerCommand>
             <lxcConfString></lxcConfString>
             <hostname></hostname>