While you urgently need to upgrade Ubuntu for other reasons, you can continue to use Certbot 0.25.0 (unless you run into bugs). While it uses TLS-SNI-01 validation by default, it supports HTTP-01 as well as more recent versions.
You can use this command to test how things will go:
sudo certbot renew --dry-run --preferred-challenges http-01,dns-01
If you really need to upgrade Certbot, you can use certbot-auto instead. But that's a small hassle (mainly you need to fix a systemd timer to use it) and IMO you should prioritize other things if possible.