USN-8447-2: LXD vulnerabilities

This update addresses Go Cryptography vulnerabilities embedded in LXD, covering CVE-2026-39830, CVE-2026-39833, CVE-2026-39834, and CVE-2026-42508. Issues include improper SSH global request handling, FIDO/U2F verification bypasses, SSH agent key constraint serialization flaws, confirm-before-use constraint enforcement failures, integer overflow in SSH channel writes, certificate authority revocation check bypasses, and source-address critical option enforcement gaps across various Ubuntu LTS versions.