Test with Go 1.21 and later

[skitt]

Something ends up requiring the slices package, which was added in Go 1.21.

See https://github.com/kubernetes-sigs/yaml/actions/runs/11232030744/job/31222775425?pr=118 for example:

Run GO111MODULE=off go get golang.org/x/lint/golint
  GO111MODULE=off go get golang.org/x/lint/golint
  shell: /usr/bin/bash -e {0}
  env:
    GOROOT: /opt/hostedtoolcache/go/1.16.15/x64
package slices: unrecognized import path "slices": import path does not begin with hostname
Error: Process completed with exit code 1.

[dims]

/approve
/lgtm

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