I just ran do-release-upgrade on one of my 20.04 servers. Some things were missing after, so I ran the apt install line from the install docs:
apt install acl curl fping git graphviz imagemagick mariadb-client mariadb-server mtr-tiny nginx-full nmap php-cli php-curl php-fpm php-gd php-gmp php-json php-mbstring php-mysql php-snmp php-xml php-zip rrdtool snmp snmpd whois unzip python3-pymysql python3-dotenv python3-redis python3-setuptools python3-systemd python3-pip
I also had to configure the php-fpm pool from the docs, since it already exists, I just copied it:
cp /etc/php/7.4/fpm/pool.d/librenms.conf /etc/php/8.1/fpm/pool.d/librenms.conf
systemctl restart php8.1-fpm
Then run ./daily.sh to switch back to the main release from the support branch (or wait for it to run)
su - librenms
./daily.sh
Double check validate for other known issues. I had to fix php timezone too.
If you have any issues compare against the install documentation. I am using nginx+php-fpm.