[v13] fix: Tidy integrations/kube-agent-updater/go.mod

[codingllama]

The go.mod file is untidy and causing problems for the lint checks.

Since #34462 (or maybe even earlier than that) the min Go version for the module, as required by its dependencies (namely github.com/sigstore/cosign/v2 v2.2.1), is Go 1.21.

[wadells]

Do you understand why lint didn't fail on #34462 and subsequent PRs? What were we missing?

[codingllama]

Fixes the issues spotted by #37679, #37875 and possibly others.

I'm not sure why this wasn't a problem until now, but I suspect it's related to our recent Go toolchain update (ie #37561).

I've checked the other go.mod files and they all seem to be tidy.

[codingllama]

I've checked the go.mods of all other active branches, for good measure. Everything seems to be OK.

Fish snippet:

find . -name go.mod | while read f
    echo $f
    pushd (dirname $f)
    go mod tidy
    popd
end

[codingllama]

It looks like this ain't passing the Rotation tests, which is another can of worms. Sadly I won't have time to purse this right now.

[wadells]

AFAICT, these test failures are unrelated to the changes herein. As someone relatively unfamiliar, I don't see what kube-agent-updater would have to do with the rotation tests. Do you have reason to believe otherwise?

[codingllama]

I don't think they have to do with this PR, but I suspect they might be more prevalent since the go update too.

[wadells]

I'm going to turn of the requirement for Unit Tests (Go) for a couple minutes to get this in.

https://github.com/gravitational/github-terraform/pull/799

At least this way linting won't block that fix when we get there.

[codingllama]

Thanks! I'll see if I can figure something out before my day ends.