[hotfix] Bump download-artifact action to v8 as GHA requires

[snuyanzin]

What is the purpose of the change

The main reason is that v5 uses NodeJS 20 which is subject to stop support from GHA
also there is a warning about that (could be seen e.g. at https://github.com/apache/flink/actions/runs/26045211542) like

Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: actions/download-artifact@v5. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026. Node.js 20 will be removed from the runner on September 16th, 2026. Please check if updated versions of these actions are available that support Node.js 24. To opt into Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment variable on the runner or in your workflow file. Once Node.js 24 becomes the default, you can temporarily opt out by setting ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Brief change log

GHA

Verifying this change

GHA
there is GHA build on my local fork where there is no warning after that https://github.com/snuyanzin/flink/actions/runs/26049895330

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes )
• The public API, i.e., is any changed class annotated with @Public(Evolving): ( no)
• The serializers: (no )
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no )
• The S3 file system connector: (no )

Documentation

• Does this pull request introduce a new feature? ( no)
• If yes, how is the feature documented? (not applicable )

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CI report:

• 3a756df Azure: SUCCESS

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