update node16 to node20 in pr-labels action

[adamancini]

• update pr-labels action to use node20

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: replicatedhq/required-workflows/.github/actions/pr-labels@main. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

• update eslint config to new flat format for eslint 9: https://eslint.org/docs/latest/use/configure/migration-guide#start-using-flat-config-files

Starting with ESLint v9.0.0, the flat config file format will be the default configuration file format. Once ESLint v9.0.0 is released, you can start using the flat config file format without any additional configuration.

• use actions/upload-artifact@v4 because https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/