Bump actions/download-artifact from 3 to 4 (#867)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 3 to 4.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](https://github.com/actions/download-artifact/compare/v3...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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/build.yml

@@ -334,7 +334,7 @@ jobs:
         run: cargo xtask book
 
       - name: Download built API docs
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: apidocs
 
@@ -534,7 +534,7 @@ jobs:
         run: rm -f .cargo/config
 
       - name: Download built dev-ver book and API docs
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: book
 
@@ -543,7 +543,7 @@ jobs:
           tar -xf book.tar
 
       - name: Download built old versions of books and API docs
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: mdbookold
 
@@ -628,7 +628,7 @@ jobs:
       - deploy
     steps:
       - name: Download books
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: bookstodeploy