Emacs, GnuTLS, and DST Root CA X3

[Jib Style]

I followed the recent announcements, but unfortunately the problem
persists.

I tested on two computers, with the following ca-certificates versions:

- ca-certificates-2.40-1
- ca-certificates-2.50-1
- ca-certificates-2.50-2
- ca-certificates-2.50-3
- ca-certificates-2.50-3 AND ca-certificates-letsencrypt-2.50-3

In all cases, the result was the same.

>From the ca-certificates-letsencrypt-2.50-3 announcement:

> It may be necessary to also remove trust for the already expired DST
> X3 root CA

I'm still trying to figure out _how_ to do this, although I'm not sure
whether it should help my situation. I'll report back with the result.

Some (non-Cygwin) Emacs users reported that GnuTLS >= 3.6.14 works.