tests: remove centos-8 support (#14056)

Centos-8 support is removed as it is EOL

.github/workflows/test.yaml

@@ -597,7 +597,7 @@ jobs:
             tests: 'tests/...'
           - group: centos
             backend: google-distro-2
-            systems: 'centos-7-64 centos-8-64 centos-9-64'
+            systems: 'centos-7-64 centos-9-64'
             tests: 'tests/...'
           - group: debian-req
             backend: google-distro-1

spread.yaml

@@ -207,10 +207,6 @@ backends:
                   workers: 6
                   storage: preserve-size
                   image: centos-7-64
-            - centos-8-64:
-                  workers: 6
-                  storage: preserve-size
-                  image: centos-8-64
             - centos-9-64:
                   workers: 6
                   storage: preserve-size
@@ -757,13 +753,6 @@ prepare: |
         restorecon -v /etc/gai.conf
     fi
 
-    if [[ "$SPREAD_SYSTEM" == centos-8-* ]]; then
-        # the default image of CentOS 8 Stream is set up in enforcing mode,
-        # which may break some tests. Note that there are tests targeting
-        # SELinux which explicitly enable enforcing mode.
-        setenforce 0
-    fi
-
     # Note that os.query or any other tool cannot be used here before the current.delta file is unpacked
     if [[ "$SPREAD_SYSTEM" == fedora-* ]]; then
         # The Fedora archive mirror seems to be unreliable.
@@ -818,11 +807,6 @@ prepare: |
         centos-7-*)
             yum install -y epel-release
             ;;
-        centos-8-*)
-            # enable powertools repository
-            dnf config-manager --set-enabled powertools
-            dnf install -y epel-release epel-next-release
-            ;;
         centos-9-*)
             dnf config-manager --set-enabled crb
             dnf install -y epel-release epel-next-release