Life is a lot easier when you upgrade more often. Ubuntu 18.04 LTS went out of support earlier this year. It is only supported via paid Extended Security Maintenance now. OwnCloud 10.2.0, which replaced ownCloud 10.1, was released almost four and a half years ago!
You will need to upgrade to PHP 7.4, but should be able to go from 10.1 to 10.13.
The recommended package to use is owncloud-complete-files . It only installs ownCloud, and does not install Apache, a database, or any of the required PHP dependencies.