Add the pg_query_canceled transient error

Context

In gitlab-org/gitlab!124438 (merged), we introduced a way to fail early backend tests in case we have a known issue slowing those tests.

The CI job is not retried in those instances, so these will be shown as broken master when they occur.

This commit is helping to reduce the noise, as we will first retry the job on master pipelines before creating a notification for it.

What does this MR do and why?

Introduce the pg_query_canceled transient error.

Expected impact & dry-runs

We should not see master-broken jobs for RSpec jobs we have stopped early (job example in an MR).

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (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