Images/CI: scan only latest Octez release

What

As the title says: we scan only the latest Octez release instead of the 21.0 and 20.3.

Why

To focus first on the latest release. We may go back to scanning multiple Octez releases in the future.

How

• We keep using parallel:matrix YAML construction in case we want to scan images of multiple Octez releases.

Manually testing the MR

Check this test pipeline: https://gitlab.com/tezos/tezos/-/pipelines/1592619450

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR