Bump actions/download-artifact from 5 to 6

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/part_docs.yml

@@ -61,7 +61,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v5
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
         with:
           name: docs
           path: docs

.github/workflows/part_test.yml

@@ -235,7 +235,7 @@ jobs:
             compile-${{ env.MIX_ENV }}-${{ runner.os }}-${{ steps.setupBEAM.outputs.otp-version }}-${{ steps.setupBEAM.outputs.elixir-version }}-
       - run: mix deps.compile
       - run: mix compile --warning-as-errors
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
         with:
           name: dialyzer_plt_dev
           path: priv/plts/