Handle existing stack label in CI labelling job

[dhruvkb]

Description

This PR

• makes the CI job 'Apply stack labels' do nothing if the stack label is already applied
• classifies utilities/ under the 'mgmt' stack so that future PRs affecting them are properly labelled automatically

Checklist

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• I ran the DAG documentation generator (if applicable).

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[AetherUnbound]

Thanks so much for the quick fixes!