website: http URL for snapshots

[Joel Brobecker]

Hi Simon,

> This page:
> 
>   https://www.gnu.org/software/gdb/current/
> 
> ... only gives a FTP URL for the snapshots.  FTP is getting harder to
> use, with modern browsers dropping support for it.  I would suggest
> adding the http URL for it (https://sourceware.org/pub/gdb/snapshots/).

I've updated the web page to point to the HTTP/S URL first, with
a link to the FTP download for those who want it.

While at it, I noticed that we still had a lot of HTTP URLs, so
I just converted them to HTTP/S as well.

-- 
Joel