Add old and new version to 'update-ubuntu-version' automation

[salasberryfin]

This PR solves issue #16450.

As a follow-up of #15772, this adds retrieving both the old and new versions of ubuntu:focal from the base image and including this information in the PR. The logic is taken from the other automations that implement this feature, e.g. update-cri-o.

[k8s-ci-robot]

Hi @salasberryfin. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[minikube-bot]

Can one of the admins verify this patch?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: salasberryfin, spowelljr

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [spowelljr]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment