Drop centos 8 stream testing

In Zed cycle testing runtime, we are targetting the centos 9 stream
- https://governance.openstack.org/tc/reference/runtimes/zed.html

With dropping the python 3.6 support, project started adding python 3.8
as minimum, example nova:
- https://github.com/openstack/nova/blob/56b5aed08c6a3ed81b78dc216f0165ebfe3c3350/setup.cfg#L13

with that, centos 8 stream job is failing 100%
- https://zuul.openstack.org/build/970d029dc96742c3aa0f6932a35e97cf
- https://zuul.openstack.org/builds?job_name=devstack-platform-centos-8-stream&skip=0

This commit drops centos-8-stream testing so that we focus on centos-9-stream.

Change-Id: I045e67b1ca79aba1b2a7be9f88d7804c69c6d781

Author: Ghanshyam Mann

.zuul.yaml

@@ -56,16 +56,6 @@
         nodes:
           - controller
 
-- nodeset:
-    name: devstack-single-node-centos-8-stream
-    nodes:
-      - name: controller
-        label: centos-8-stream
-    groups:
-      - name: tempest
-        nodes:
-          - controller
-
 - nodeset:
     name: devstack-single-node-centos-9-stream
     nodes:
@@ -146,36 +136,6 @@
         nodes:
           - compute1
 
-- nodeset:
-    name: openstack-two-node-centos-8-stream
-    nodes:
-      - name: controller
-        label: centos-8-stream
-      - name: compute1
-        label: centos-8-stream
-    groups:
-      # Node where tests are executed and test results collected
-      - name: tempest
-        nodes:
-          - controller
-      # Nodes running the compute service
-      - name: compute
-        nodes:
-          - controller
-          - compute1
-      # Nodes that are not the controller
-      - name: subnode
-        nodes:
-          - compute1
-      # Switch node for multinode networking setup
-      - name: switch
-        nodes:
-          - controller
-      # Peer nodes for multinode networking setup
-      - name: peers
-        nodes:
-          - compute1
-
 - nodeset:
     name: openstack-two-node-centos-9-stream
     nodes:
@@ -694,16 +654,6 @@
 # we often have to rush things through devstack to stabilise the gate,
 # and these platforms don't have the round-the-clock support to avoid
 # becoming blockers in that situation.
-- job:
-    name: devstack-platform-centos-8-stream
-    parent: tempest-full-py3
-    description: CentOS 8 Stream platform test
-    nodeset: devstack-single-node-centos-8-stream
-    voting: false
-    timeout: 9000
-    vars:
-      configure_swap_size: 4096
-
 - job:
     name: devstack-platform-centos-9-stream
     parent: tempest-full-py3
@@ -877,7 +827,6 @@
         - devstack-ipv6
         - devstack-enforce-scope
         - devstack-platform-fedora-latest
-        - devstack-platform-centos-8-stream
         - devstack-platform-centos-9-stream
         - devstack-platform-debian-bullseye
         - devstack-multinode