Fix handling of PowerTools repo on CentOS 8. (#10344)

They appear to have changed DNF to be case-sensitive for repo naming,
and the repo name that is expected is all lower case.

packaging/installer/install-required-packages.sh

@@ -1550,7 +1550,7 @@ validate_tree_centos() {
     echo >&2 " > Checking for PowerTools ..."
     if ! run yum ${sudo} repolist | grep PowerTools; then
       if prompt "PowerTools not found, shall I install it?"; then
-        run ${sudo} yum ${opts} config-manager --set-enabled PowerTools
+        run ${sudo} yum ${opts} config-manager --set-enabled powertools
       fi
     fi
 

packaging/installer/methods/manual.md

@@ -164,7 +164,7 @@ And install the minimum required dependencies:
 yum install -y 'dnf-command(config-manager)'
 
 # Enable PowerTools
-yum config-manager --set-enabled PowerTools
+yum config-manager --set-enabled powertools
 
 # Enable EPEL
 yum install -y epel-release