Use debian current stable for dib

The current stable release for debian is bullseye, this patch switches
dib-based images to that version.

Change-Id: Id607d7697c0aa8464b533047a4e5938648a0e25b

doc/source/admin/dib.rst

@@ -5,7 +5,7 @@ Images built using diskimage-builder_ are recommended for production use on
 real hardware. The recommended distributions are:
 
 * CentOS 8 (regular and stream)
-* Debian Buster (``debian-minimal`` element)
+* Debian Bullseye (``debian-minimal`` element)
 
 The following should work but receive only limited testing and support:
 

zuul.d/ironic-python-agent-builder-jobs.yaml

@@ -38,7 +38,7 @@
     vars:
       image_type: 'dib'
       image_distro: 'debian-minimal'
-      image_release: 'buster'
+      image_release: 'bullseye'
 
 - job:
     name: ironic-python-agent-check-image-base
@@ -128,7 +128,7 @@
     vars:
       image_type: 'dib'
       image_distro: 'debian-minimal'
-      image_release: 'buster'
+      image_release: 'bullseye'
 
 - job:
     name: ironic-python-agent-check-image-dib-debian-arm64