Website: update the outdated GDC download link.

[dlangBugzillaToGithub]

mw reported this on 2020-05-18T17:10:14Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20841

Description

On the page:

https://dlang.org/download.html

The GDC download link direct to:

https://gdcproject.org/downloads

Where the information is very outdated, still back to 2016!

E.g. I searched Google for gdc-10_10.1.0-1_amd64.deb

even this mirror site is better:

https://mirror.qlnet.ch/debian/pool/main/g/gcc-10/