bump: Switch to libssh v0.12.1

[david-cermak]

From upstream changelog

version 0.12.1 (released 2026-07-21)

• Security:
  • CVE-2026-15370: Stack buffer overflow in SFTP server longname construction
  • CVE-2026-59842: Information disclosure via short GSSAPI Curve25519 public key
  • CVE-2026-59843: Denial of service via zero advertised channel packet size
  • CVE-2026-59844: Denial of service via oversized SFTP read length
  • CVE-2026-59845: Denial of service via unchecked ProxyCommand fork() failure
  • CVE-2026-59846: Information disclosure via ProxyCommand %r username expansion
  • CVE-2026-59847: Integrity downgrade via OpenSSL AES-GCM tag verification
  • CVE-2026-59848: Denial of service via SFTP responses with unknown request IDs
  • CVE-2026-59849: Denial of service via automatic certificate authentication loop
  • CVE-2026-59850: Use-after-free via data callbacks on closed channels
  • CVE-2026-59851: Authentication bypass via missing GSSAPI principal check
  • Zero-initialize every ssh_string
• Compatibility:
  • Fix compatibility with C23 / gcc16
  • Allow hybrid ML-KEM key exchange in FIPS mode
• Bugfixes:
  • Fix multiple memory leaks, null checks, and error checks
  • Fix parameter size mismatch in mlkem768x25519-sha256
  • Fix client SFTP messages being ignored if sent at high rate
  • Validate peer public key in DH key exchange
  • Fix ambiguous error reporting of sftp_init
  • Fix hidden integer underflow in socket packet callback
  • Avoid remote window overflow
  • Fix socket data callback return value on rekey failure
  • Avoid off-by-one overflow during kbdint authentication
  • Avoid logging uninitialized sequence numbers
  • Avoid double conversion of SFTP version number
  • Send correct SFTP server version number
  • Avoid handling repeated SFTP INIT messages
  • Harmonize return values from SFTP server callbacks