Add whats_new entry for GitLab 19.0

What does this MR do?

Adds the whats_new YAML file for GitLab 19.0 (data/whats_new/202605210001_19_00.yml), following the template at data/whats_new/templates/YYYYMMDD0001_XX_YY.yml and the 18.11 example at data/whats_new/202604160001_18_11.yml.

All Primary features from the 19.0 release post are included, along with items from the following in-flight MRs:

• !236095 (merged) (Configure work item types)
• !236001 (merged) (Dependency scanning by using SBOM generally available)

Features included

1. Group-level custom review instructions for GitLab Duo - Premium, Ultimate
2. Configure work item types - Premium, Ultimate (from !236095 (merged))
3. GitLab Secrets Manager now available in open beta - Premium, Ultimate
4. GitLab Duo Developer enhancements for merge request workflows - Free, Premium, Ultimate
5. Dependency scanning by using SBOM generally available - Ultimate (from !236001 (merged))

Related issues

#597533

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.