Move to go 1.20

[BobKerns]

At least 1.18 is required to build the current source, to support the file embedding.

1.16 is the earliest with the feature, but the all:mgmt pattern doesn't work until 1.18.

(I might also suggest adding a Docker build of at least one platform to the release script. It would catch issues like this).

[bk2204]

Hey, please don't add unhelpful approvals and comments to existing PRs. If you don't stop, we'll have to block you.