Allow ticket gardener to run gitlab-triage via GitLab CI

(Extracting ideas from #17740 (closed) so we can eventually close that issue.)

Currently, our ticket gardener regularly runs gitlab-triage on their own machine. It can take hours due to #17823 (closed), which is inconvenient (that machine must remain up and online during that time). But even if took less long, this operation requires more Linux sysadmin skills than it should, and a GitLab API token for the root user. All these problems would be solved by running gitlab-triage via GitLab CI on a runner we trust enough to give it such privileges.

cc @zen

Note: For S11, this fits in:

• D.1. Make the Tails development process more efficient: Triaging tickets is fundamental for coordination of development work and improving this process has positive impact on the efficiency of the development process.

To-do

• Run gitlab-triage in CI
• Fix documentation accordingly → tails!1606 (merged)