Add GeoServer SSRF advisory references

[cookesan]

Adds GeoServer SSRF advisory references to GHSA-x4r9-gmw3-hxww.

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-x4r9-gmw3-hxww/GHSA-x4r9-gmw3-hxww.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-x4r9-gmw3-hxww and CVE-2025-58175 remain public and consistent with this advisory.
• Verified geoserver/geoserver PR Hook #8622, backport PRs #8753 and #8754, commits b85ff6df2ad7ca30fd23e31c15fa504ecb1b31e5 and 68474568a92cc37cf12884efe648946e65407946, and releases 2.26.4 and 2.27.3.
• Added the missing backport PR references plus the NVD advisory reference and nvd_published_at value after confirming the public records.
• Local validation passed: jq empty and git diff --check.

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