Add 19.4 release note for GitLab flow builder (Beta)

James Caseyrequested to merge
docs/19-3-gitlab-flow-builder-beta into master

What does this MR do?

Adds the GitLab 19.4 release note for the GitLab flow builder (Beta) in VS Code

Related issues

Blocked on

Waiting on docs MR !250171 (merged), which still has nine open tech-writer questions (beta/GA status, version numbers, prerequisites, terminology, and builder capabilities). Do not merge before that lands, since this note's accuracy depends on those answers.

Edited by Uma Chandran (OOO Aug 28 - Sept 1)

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