Build our gerrit images on Bullseye

This is a bit of spring cleaning. Previously we based on images on
Buster but Bullseye exists now so give it a go.

Change-Id: Icc3d79b361e41df2f2f063993fd206ab7d992f75

docker/gerrit/base/Dockerfile

@@ -15,12 +15,12 @@
 
 # Fri Aug 20 22:55:30 UTC 2021 - trigger rebuild
 
-FROM docker.io/opendevorg/python-builder:3.7 as builder
+FROM docker.io/opendevorg/python-builder:3.9-bullseye as builder
 
 COPY . /tmp/src
 RUN assemble
 
-FROM docker.io/library/openjdk:11 as gerrit-base
+FROM docker.io/library/openjdk:11-bullseye as gerrit-base
 
 RUN echo 'APT::Install-Recommends "0";' > /etc/apt/apt.conf.d/95disable-recommends
 

zuul.d/docker-images/gerrit.yaml

@@ -6,7 +6,7 @@
     required-projects: &gerrit_base_projects
       - opendev/jeepyb
       - opendev/system-config
-    requires: python-builder-3.7-buster-container-image
+    requires: python-builder-3.9-bullseye-container-image
     provides: gerrit-base-container-image
     pre-run: &gerrit-base_prerun
       - playbooks/zuul/gerrit/base.yaml
@@ -24,7 +24,7 @@
     name: system-config-upload-image-gerrit-base
     description: Build and upload a gerrit-base image.
     parent: system-config-upload-image
-    requires: python-builder-3.7-buster-container-image
+    requires: python-builder-3.9-bullseye-container-image
     provides: gerrit-base-container-image
     pre-run: *gerrit-base_prerun
     required-projects: *gerrit_base_projects