CI: remove [scan] stage

Follow-up of

1. !18254 (merged) (remove [doc] stage)
2. !18263 (merged) (streamline publishing related stages)

What

Removes [scan] stage, used in scheduled container scanning pipelines.

Why

• Scanning an image is testing that it contains no known vulnerabilities.
• Having less stages make pipelines easier to visualise and understand.

How

Self-evident.

• Remove the [scan] stage.
• Move relevant jobs to the [test] stage.
• make -C ci

Manually testing the MR

• make -C ci check
• schedule_container_scanning_master test pipeline: https://gitlab.com/tezos/tezos/-/pipelines/1874432973

Next steps

• In the end, we should only have a few core stages: start, build, test, publish (and perhaps [images] at first).
• We are planning to progressively remove all other stages and dispatch their jobs into these core stages.
• Next stages to be removed:
  • sanity
  • publish_tests
  • manual

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