CI: put all trigger jobs in [start] stage

What

1. Moves all [trigger] jobs in [start] stage.
2. Enforces this in the definition of [Tezos_ci.trigger_job]

Why

1. a) Consistent semantics of the stages. b) Consistent with Cacio. c) Clearer pipelines when using the Stages view.
2. To prevent defining trigger jobs in other stages.

How

1. Look for all usages of trigger_job and modify the stage + make -C ci.
2. Modification is done at the Tezos_ci level, not in Gitlab_ci: the latter is lower-level and should reflect what can be expressed in Gitlab CI/CD YAML. And in Gitlab CI/CD YAML, trigger jobs can be in any stage.

Note that the second commit does not modify the CI yaml files.

Manually testing the MR

• make -C ci check
• This is just a stage change. Given that we move trigger jobs to the left, we shouldn't break the rule that every job must depend on a job not in a later stage.
• Test pipeline: the [before_merging] pipeline of this stage: https://gitlab.com/tezos/tezos/-/pipelines/1927317282. (Compare with equivalent pipeline before the MR: https://gitlab.com/tezos/tezos/-/pipelines/1902466081)

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