Devbuild site: Set GZDoom forum link to ZDoom forums

[Major Cooke]

The devbuilds site still has the GZDoom forums link set as the DRD forums, despite closure. I think it'd be prudent to change the link to point at ZDoom forums instead, or to "What happened to the GZDoom and QZDoom forums?" thread.

The former is recommended in the event the DRD team forums are shut down for any reason so people won't be lost.