update to Go 1.18.x

[galargh]

Release notice: https://go.dev/blog/go1.18

Open discussions on how to handle Go 1.18:

• Suggestion: drop Go 1.17 when Go 1.18 is released #258
• update to Go 1.18 #193

From what I understand, we haven't reached a consensus on changing the pattern of supporting latest 2 Go versions. That's why I propose this update as-is.

I'd really appreciate if we could do the next major unified CI release with #261 because it'd give us a chance that the release process itself succeeds without human intervention.

Finally, the latest released version of staticcheck does not support Go 1.18 yet. The progress is tracked in dominikh/go-tools#1166 and, apparently, there were comments in some other issues that this is going to be complete in about a week. The question is: do we wait for staticcheck release or disable it in unified CI for now?

Testing

• Updated .github-test-target in sync: update CI config files .github-test-target#37 (this also showcases the staticcheck issue)

[mvdan]

I don't think we need to block adding Go 1.18 until the next staticcheck version is out. The main reason that staticcheck is broken with 1.18 is due to generics, but the vast majority of repos using unified CI will keep on supporting Go 1.17, meaning they generally can't use generics yet, unless they go for some funky business with //go:build go1.18.

This assumes that it will be painless to update staticcheck in a week's time once they do release proper support for Go 1.18, though. I assume that for most repos it would be a bit bothersome to merge the update to Go 1.18 this week, and a staticcheck update next week. For that reason, it might be best to wait a week until we deploy the Go 1.18 update.

[galargh]

I just realised this duplicates #307.