security/putty: SECURITY update to 0.81

This fixes a vulnerability where NIST P521 ecdsa-sha2-nistp521 (only!)
keys can be recovered easily because signature generation in PuTTY used
a biased random number generator.

MFH: 2024Q2
Security: 080936ba-fbb7-11ee-abc8-6960f2492b1d
Security: CVE-2024-31497