build: switch tested centos base images (#20417) (#20418)

(cherry picked from commit 2e26dc62cb)

releng/packages/spec/clean_install/_install_uninstall.bash

@@ -27,7 +27,7 @@ function testInstalled() {
 
 function testUninstalled() {
   if command -v "$1" >/dev/null 2>&1 ; then
-    >&2 echo "$1 still on \$PATH after install"
+    >&2 echo "$1 still on \$PATH after uninstall"
     exit 1
   fi
 }

releng/packages/spec/clean_install/run.bash

@@ -63,7 +63,7 @@ fi
 
 if [ -n "$IS_RPM" ]; then
   # Latest is the most recent LTS, and Rolling is the most recent release.
-  for BASE_IMAGE in centos:6 centos:7 ; do
+  for BASE_IMAGE in centos:7 centos:8 ; do
     if [ -n "$PKG" ]; then
       dockerTest "$PKG" /data.rpm -R
     fi