Update workload registrar link on the downloads page

[evan2645]

The downloads page links to the kubernetes workload registrar, but the registrar has been deprecated and the link is now broken. Instead, we should link to the spire-controller-manager, which has superseded the kubernetes workload registrar.

[unnathik]

I'd like to work on this!

[sanderson042]

Hi @unnathik - I appreciate your offer to help! This may not be a good use of your time because the Downloads page is broken in a more substantial way as noted in bug #276. As described in that bug, it needs some major restructuring. Some other simple documentation bugs that might be a better use of your time are #213 or #232. Or I can help you restructure the Downloads page if you want a more challenging task.

I'll remove the help wanted and good first issue tags from this bug.

Thank you! 🙏