Drop centos 8 stream jobs

CentOS 8 Stream has reached its end of life and the upstream
package repos have been removed. The CentOS nodes from Zuul and
Nodepool are being removed. See the following email announcement:

https: //lists.opendev.org/archives/list/service-announce@lists.opendev.org/thread/YHPRWKWULKJATAJZPFIZCBHS7HZJE5V2/
Change-Id: Id043757e970880c1576e1b8fc8472477e4dd8a51

.zuul.yaml

@@ -10,15 +10,10 @@
       zuul_work_dir: '{{ zuul.project.src_dir }}'
       zuul_work_virtualenv: '{{ ansible_user_dir }}/.venv'
 
-- job:
-    name: bindep-centos-8-stream
-    parent: bindep-base
-    nodeset: centos-8-stream
-
 - job:
     name: bindep-centos-9-stream
     parent: bindep-base
-    nodeset: centos-8-stream
+    nodeset: centos-9-stream
 
 - job:
     name: bindep-ubuntu-bionic
@@ -42,7 +37,6 @@
       - publish-opendev-nox-docs
     check:
       jobs:
-        - bindep-centos-8-stream
         - bindep-centos-9-stream
         - bindep-ubuntu-bionic
         - bindep-ubuntu-focal
@@ -58,7 +52,6 @@
         - nox-cover
     gate:
       jobs:
-        - bindep-centos-8-stream
         - bindep-centos-9-stream
         - bindep-ubuntu-bionic
         - bindep-ubuntu-focal