move v1.21 from netlify to gh like in toml file

[acornett21]

Description of the change:
Updating config.toml for v1.21 to point to github, instead of netlify route.

Motivation for the change:
We have a limited number of routes in netlify, so need to remove one route before the next release.

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs