CI/Docs: use [gcp_very_high_cpu] in [docgen, odoc]

What

• Change the runner used in [documentation.odoc] and [documentation.docgen] from gcp to gcp_very_high_cpu.

Why

To decrease wall-time of [merge_train] and [before_merging] pipelines.

• In our tests, we gain about 6 min in these jobs by switching runners (script_step duration).
  • The current average duration is about 10 min (cf. datadog).
  • So we should expect an average duration of 4 min.
  • Given that the cost of gcp_very_high_cpu is about 2x the cost of gcp, we should even expect to decrease costs as well.
• This should remove these jobs from the critical path. Instead the bottleneck will be octez oc.build_* jobs as for merge_train pipelines w/o doc jobs.
• This means that we should save the difference between the current max duration of [documentation.odoc] and [documentation.docgen] and the max duration of oc.build_* jobs. There is a lot of variance (due e.g. to the time to start the pod), but it looks like the save should be around 3 min.

NB: we tried using gcp_very_high_cpu for other documentation jobs (documentation.manuals and documentation.build_all) but the gains were non-existent, as the script step duration is < 10 s, the bulk of the job duration is likely to be the time it takes to start the pod.

How

• Simple change in docs/ci and then generate the yaml.

Manually testing the MR

• make -C ci check should pass
• look at these data in datadog on a poc branch 8094-bruno-doc-gcp_very_high_cpu(in the first test pipeline, documentation.docgen was still using gcp).

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

Relates to #8094