build(deps): bump actions/download-artifact from 1 to 4

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 1 to 4.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](https://github.com/actions/download-artifact/compare/v1...v4)

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

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/nightly_docs.yml

@@ -44,7 +44,7 @@ jobs:
       - name: Remove old latest
         run: rm -rf ./docs/.vuepress/public/docs-rs/bdk/nightly/latest
       - name: Download built docs
-        uses: actions/download-artifact@v1
+        uses: actions/download-artifact@v4
         with:
           name: built-docs
           path: ./docs/.vuepress/public/docs-rs/bdk/nightly/latest