Add Gitea release attachment fix references

[cookesan]

Adds Gitea release attachment fix references to GHSA-4xx9-vc8v-87hv.

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/2026/01/GHSA-4xx9-vc8v-87hv/GHSA-4xx9-vc8v-87hv.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-4xx9-vc8v-87hv and CVE-2026-20912 remain public and consistent with this advisory.
• Verified go-gitea/gitea PRs #36320, #36355, #36347, and #36375, commits fbea2c68e8df11cfa94e8ead913b79946780ed30 and dfe4055b928b4e14a1835582bcf2936b6018c224, release v1.25.4, and the Go module proxy record for github.com/go-gitea/gitea v1.25.4.
• Local validation passed: jq empty and git diff --check.

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