Bump actions/download-artifact from 6 to 8 (#2742)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 6 to 8.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v6...v8)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '8'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Merge pull request #2697 from su2code/release_v8.4.0

Changing version number to 8.4.0

Merge pull request #2559 from su2code/release_v8.3.0

Changing version number to 8.3.0

Merge pull request #2498 from su2code/release_v8.2.0

Changing version number to 8.2.0

Merge pull request #2359 from su2code/release_v8.1.0

Changing version number to 8.1.0

Merge pull request #2223 from su2code/release_v8.0.1

Changing version number to 8.0.1

Merge pull request #2120 from su2code/release_v8.0.0

Changing version number to 8.0.0

Merge pull request #1912 from su2code/release_v7.5.1

Changing version number to 7.5.1

Merge pull request #1848 from su2code/release_v7.5.0

Changing version number to 7.5.0

Merge pull request #1735 from su2code/release_v7.4.0

Changing version number to 7.4.0