ci/gha: bump go 1.16-rc1 -> 1.16.x; fix gha ci

[kolyshkin]

As the final go 1.16 is released, rc1 is no longer available.

This fixes our CI.

[kolyshkin]

Can we merge this one quick please? Currently the CI is broken as go-1.16-rc1 is removed from downloads.

[mrunalp]

@AkihiroSuda ptal

[kolyshkin]

Marked go 1.16.x jobs as required

[cyphar]

Can we switch to 1.x and tip (or beta) rather than having to update the GHA with each Go release? We might want to keep some older versions in the GHA but I'd prefer to not have to do this every time if possible.

[thaJeztah]

We might want to keep some older versions in the GHA

Those older versions would have to be updated if a new Go version is released (assuming support is "latest + previous"), or did you want to run "latest" + <some fixed minimal version>" ?

I'm slightly concerned that 1.x makes it somewhat unpredictable what version a pull request ran on (and adding support for a new Go version may still need to be a conscious decision)

[cyphar]

IMHO we should be testing whether CI fails if you build HEAD with a new Go version, we don't really have the concept of a "supported Go version" in runc in the same way that exists in Docker. The issue is we probably want to have latest + previous (maybe + one fixed minimal version), but I think having 1.x strictly improves the situation even if we end up regularly updating the GHA configuration -- at the very least we immediately know if a new Go version breaks runc.