Update php installation instruction for CentOS 8 to reflect the minim…

…um php version now being 8.1 (#14159)

doc/Installation/Install-LibreNMS.md

@@ -38,16 +38,18 @@ Connect to the server command line and follow the instructions below.
     === "NGINX"
         ```
         dnf -y install epel-release
+        dnf -y install dnf-utils http://rpms.remirepo.net/enterprise/remi-release-8.rpm
         dnf module reset php
-        dnf module enable php:7.3
+        dnf module enable php:8.1
         dnf install bash-completion cronie fping git ImageMagick mariadb-server mtr net-snmp net-snmp-utils nginx nmap php-fpm php-cli php-common php-curl php-gd php-gmp php-json php-mbstring php-process php-snmp php-xml php-zip php-mysqlnd python3 python3-PyMySQL python3-redis python3-memcached python3-pip python3-systemd rrdtool unzip
         ```
 
     === "Apache"
         ```
         dnf -y install epel-release
+        dnf -y install dnf-utils http://rpms.remirepo.net/enterprise/remi-release-8.rpm
         dnf module reset php
-        dnf module enable php:7.3
+        dnf module enable php:8.1
         dnf install bash-completion cronie fping git httpd ImageMagick mariadb-server mtr net-snmp net-snmp-utils nmap php-fpm php-cli php-common php-curl php-gd php-gmp php-json php-mbstring php-process php-snmp php-xml php-zip php-mysqlnd python3 python3-PyMySQL python3-redis python3-memcached python3-pip python3-systemd rrdtool unzip
         ```