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: If00058451b89b93ed5b63ce4d6ade5f85785fc50

.zuul.yaml

@@ -19,10 +19,8 @@
         - openstack-tox-py310
         - openstack-tox-py311
         - openstack-tox-py312
-        - pyeclib-tox-py36-centos-8-stream
         - pyeclib-tox-py39-centos-9-stream
         - libec-pyeclib-unit
-        - libec-pyeclib-unit-centos-8-stream
         - libec-pyeclib-unit-centos-9-stream
     gate:
       jobs:
@@ -41,10 +39,8 @@
         - openstack-tox-py310
         - openstack-tox-py311
         - openstack-tox-py312
-        - pyeclib-tox-py36-centos-8-stream
         - pyeclib-tox-py39-centos-9-stream
         - libec-pyeclib-unit
-        - libec-pyeclib-unit-centos-8-stream
         - libec-pyeclib-unit-centos-9-stream
     pre-release:
       jobs:
@@ -57,13 +53,6 @@
             vars:
               build_wheel: false
 
-- job:
-    name: pyeclib-tox-py36-centos-8-stream
-    parent: openstack-tox-py36
-    description: |
-      Run unit tests using ``py36`` environment on CentOS 8.
-    nodeset: centos-8-stream
-
 - job:
     name: pyeclib-tox-py39-centos-9-stream
     parent: openstack-tox-py39
@@ -71,14 +60,6 @@
       Run unit tests using ``py39`` environment on CentOS 9.
     nodeset: centos-9-stream
 
-- job:
-    name: libec-pyeclib-unit-centos-8-stream
-    parent: openstack-tox-py36
-    timeout: 1800
-    nodeset: centos-8-stream
-    vars:
-      tox_envlist: py36-compilelibs
-
 - job:
     name: libec-pyeclib-unit-centos-9-stream
     parent: openstack-tox-py39