Draft: Clarify stuck_or_timeout_failure retry reasons

What does this MR do?

Updates the retry:when documentation and the GitLab 19.0 release notes to reflect the new, more specific CI job failure reasons that replace the generic stuck_or_timeout_failure value, as introduced by !230787 (merged) and described in #595752 (closed).

Changes

1. doc/ci/yaml/_index.md — Updates the retry:when keyword reference:
   • Adds a note clarifying that stuck_or_timeout_failure is preserved for historical data only.
   • Documents the new failure reasons:
     • stuck_pending_with_matching_runners
     • stuck_pending_no_matching_runners
     • no_updates_running
     • no_updates_canceling
     • server_timeout_running
     • server_timeout_canceling
   • Updates the array example for retry:when to use the new, more specific values.
2. doc/releases/19/gitlab-19-0-released.md — Adds a 19.0 release note entry under Unified DevOps and Security describing the new failure reasons.

Related issues and MRs

• Documents change introduced by !230787 (merged).
• Addresses the documentation acceptance criterion in #595752 (closed).
• Related to the broader job failure reason standardization in #595703 (closed).

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
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  • Assign the designated Technical Writer.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Codeowners.

• If you're adding or changing the structure of the documentation, the SSOT navigation must be updated.
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