Update exporting_for_dedicated_servers.rst

[mlgarchery]

Link was dead, made it point to https://godotengine.org/download/3.x/server/.
What should be done for v4 ? We don't have the Linux Server option we had on v3:

[skyace65]

The 4.x Linux binary can be run with the --headless option which is the equivalent of the 3.x server/headless build, which also means a linux server download on the site isn't coming back. You can update this PR to remove the link to the server download and add that information.

That said the 3.6 branch of the docs does need that link updated to "https://godotengine.org/download/3.x/server." If you want you can make a separate PR doing that which targets the

[mhilbrunner]

Superseded by #7701, which documents the new --headless way of doing this. Thanks for contributing, sorry this PR took some time to get a response, we were quite busy with everything going on currently :) Still, thanks for contributing!

[mhilbrunner]

@mlgarchery Ported this to the 3.x branch, where it is still relevant, giving you credit on the commit: #7713

I'll also see if we can create a redirect on the webserver to fix broken external links. Thanks!