Add go1.21, use for go1.21.0

[joshwlewis]

This adds go1.21.0, and uses go1.21.0 for go1.21. It also adds a simple test for the go1.21 line.

Supersedes #521.

[edmorley]

The change looks good, but CI is failing with curl: (6) Could not resolve host: null even on retry - an unrelated issue, but strange nonetheless?

[edmorley]

Oh, it seemed unrelated because there were a dozen error lines (so it seemed multiple assets were failing the same way), but I guess the additional lines were just retries, so they were all from the newly added assets - whose URLs have just been fixed in the followup commits?

[joshwlewis]

The change looks good, but CI is failing with curl: (6) Could not resolve host: null even on retry - an unrelated issue, but strange nonetheless?

I think this has to do with an upstream artifact naming change. Traditionally, the first release doesn't have a .0 suffix for go releases. For example, the first go1.20 was go1.20 (no .0), and go1.20.0 doesn't exist in the upstream distribution repository. For some reason, there is a suffix on this release - go1.21 doesn't exist upstream, but go1.21.0 does.

[joshwlewis]

It looks like the .0 is intentional: golang/go#57631