Backport 19.3 release notes to 19-3-stable-ee

What does this MR do and why?

This backports the GitLab 19.3 release notes from master to 19-3-stable-ee.

The release notes were finalized on master after the stable branch was cut, so the copy on the stable branch is out of date. This MR brings the two in line:

• index.md is updated from the pre-release wording ("GitLab 19.3 (not yet released)") to the published version, including the release date of 2026-08-20.
• Six feature entries that were added to master after the branch cut are copied over:
  • bulk_flow_execution.md
  • flow-creator-foundational-agent.md
  • sd-default-branch-commit-range-scan.md
  • sd-gss-beta.md
  • spdx-license-expression-sbom-ingestion.md
  • vulnerability_details_redesign.md

The file contents are copied verbatim from master. This is a documentation-only change with no code, spec, or configuration changes.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.