Testing migration path for major upgrades - bump to testing 12->13 upgrades.

What does this MR do?

We are testing the migration upgrade path for major version upgrades in CI. For major upgrade, we recommend (or require?) an installation to go to the latest minor version for the currently running major version first. Only then upgrade across to the next major version: https://docs.gitlab.com/ee/policy/maintenance.html#upgrade-recommendations

Now in CI config, we are currently testing 11.11.0 -> latest version. This MR changes it to start with 12.10.0 instead.