GitHub suggested changes commit message doesn't get DCO

[scottrigby]

Re https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/incorporating-feedback-in-your-pull-request

When a PR author is prompted to "commit suggestion" the commit message textarea should be populated with the configured DCO signoff.

I see this happened in helm/chart-releaser#72 and was brought to my attention by @unguiculus - thanks!

I noticed when resolving a merge conflict GitHub UI gives no text box, but it seems DCO bot ignores merge commits anyway so, that's probably OK?

If anyone notices other places we can target please lmk 💖