Backport of docs: revert to labels={"foo.bar": "baz"} style into release/1.8.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #26535 to be assessed for backporting due to the inclusion of the label backport/1.8.x.

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The below text is copied from the body of the original PR.

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Back in #24074 I thought it was necessary to wrap labels in a list to support quoted keys in hcl2. This... doesn't appear to be true at all? The simpler labels={...} syntax appears to work just fine.

I updated the docs and a test (and modernized it a bit). I also switched some other examples to the labels = {} format from the old labels{} format.

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