chore(deps): 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/build.yml

@@ -187,7 +187,7 @@ jobs:
 
       - name: Download built QEMU
         if: contains(matrix.input.platform, 'esp32')
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: qemu
 
@@ -331,7 +331,7 @@ jobs:
         run: cargo xtask book
 
       - name: Download built API docs
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: apidocs
 
@@ -531,7 +531,7 @@ jobs:
         run: rm -f .cargo/config
 
       - name: Download built dev-ver book and API docs
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: book
 
@@ -540,7 +540,7 @@ jobs:
           tar -xf book.tar
 
       - name: Download built old versions of books and API docs
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: mdbookold
 
@@ -625,7 +625,7 @@ jobs:
       - deploy
     steps:
       - name: Download books
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: bookstodeploy