Update upgrade-ubuntu article for 24.04 · system76/docs@882ee33

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The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2, actions/setup-node@v2, xt0rted/markdownlint-problem-matcher@v1. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

The following actions use a deprecated Node.js version and will be forced to run on node20: actions/checkout@v2, actions/setup-node@v2, xt0rted/markdownlint-problem-matcher@v1. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

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