Add Angular platform-server advisory references

[cookesan]

Adds Angular platform-server advisory references to GHSA-gxx4-3xcv-f8qx.

Audit refresh, July 8, 2026:

• Confirmed the live PR is open, ready for review, merge-clean, and the visible advisory check is green.
• Rechecked the merge-base diff: exactly one advisory JSON file changes (advisories/github-reviewed/2026/06/GHSA-gxx4-3xcv-f8qx/GHSA-gxx4-3xcv-f8qx.json), JSON parses, git diff --check passes, and the exact-head worktree is clean.
• Revalidated the added public references in the diff; all resolve through their source site or registry.

[cookesan]

Refreshed on July 8, 2026:

• GHSA-gxx4-3xcv-f8qx and CVE-2026-50556 remain public and consistent with this advisory.
• Verified Angular issue #68903, angular/domino PR What's the plan for self-published advisories? #29, commit f74cccd496283b6141fbdbeb25b168f4b1c9836a, releases v19.2.25, v20.3.24, v21.2.16, and v22.0.0-rc.2, and the matching npm @angular/platform-server versions.
• Added the NVD advisory reference and nvd_published_at value after confirming the public NVD record.
• Local validation passed: jq empty and git diff --check.

No affected range, package, or severity changes are included.