Bump actions/download-artifact from 3 to 4

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 3 to 4.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v3...v4)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/ftl-build.yml

@@ -150,7 +150,7 @@ jobs:
         uses: actions/checkout@v4
       -
         name: Download digests
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: digests
           path: /tmp/digests