ci: Use GITHUB_OUTPUT envvar instead of set-output command · mullvad/mullvadvpn-app@33041e9

The following actions use a deprecated Node.js version and will be forced to run on node20: actions/checkout@v3. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

Run yamllint .: gui/crowdin.yml#L5

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Run yamllint .: ios/.swiftlint.yml#L10

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