Specify that 'deploy:secrets' is a deployment preparation step

What does this MR do and why?

I noticed that deploy:secrets was listed in the "deployment" jobs here: https://gitlab.com/gitlab-org/quality/triage-ops/-/environments/8138926

There's a way to specify that this job is actually a preparatory job, see https://docs.gitlab.com/ee/ci/yaml/#environmentaction.

Expected impact & dry-runs

https://gitlab.com/gitlab-org/quality/triage-ops/-/environments/8138926 should only list the deploy-triage-web jobs.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review