putty -- add protocol extension against 'Terrapin attack'

Simon Tatham reports:

PuTTY version 0.80 [contains] one security fix […] for a newly discovered security issue known as the ‘Terrapin’
attack, also numbered CVE-2023-48795. The issue affects widely-used
OpenSSH extensions to the SSH protocol: the ChaCha20+Poly1305
cipher system, and ‘encrypt-then-MAC’ mode.
In order to benefit from the fix, you must be using a fixed version
of PuTTY and a server with the fix, so that they can agree to
adopt a modified version of the protocol. […]