Check out this article: CentOS Linux EOL
The below commands helped me:
sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-Linux-*
sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-Linux-*
Doing this will make DNF work, but you will no longer receive any updates.
To upgrade to CentOS 8 stream:
sudo dnf install centos-release-stream -y
sudo dnf swap centos-{linux,stream}-repos -y
sudo dnf distro-sync -y
Optionally reboot if your kernel updated (not needed in containers).