bug#66304: exim vulnearable to CVE-2023-42115 et al

[Wilko Meyer]

Hi Guix,

Exim currently has unpatched vulnearabilities regarding its EXTERNAL
Auth driver as well as its SPA/NTLM authenticator.

According to the project[0] prospective fixes seem to be around the
corner. We should probably bump the Exim version we ship to a
non-vulnearable version as soon as one is available.

[0]: https://www.exim.org/static/doc/security/CVE-2023-zdi.txt

-- 
Kind regards,

Wilko Meyer
w@wmeyer.eu