maint: 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/ci_cd.yml

@@ -63,7 +63,7 @@ jobs:
     steps:
 
       - name: "Download HTML documentation artifacts"
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: documentation-html
           path: documentation-html
@@ -84,7 +84,7 @@ jobs:
     steps:
 
     - name: "Download HTML documentation"
-      uses: actions/download-artifact@v5
+      uses: actions/download-artifact@v6
       with:
         name: documentation-html
         path: documentation-html
@@ -96,7 +96,7 @@ jobs:
         dest: documentation-html.zip
 
     - name: "Download PDF documentation"
-      uses: actions/download-artifact@v5
+      uses: actions/download-artifact@v6
       with:
         name: documentation-pdf
         path: documentation-pdf
@@ -125,7 +125,7 @@ jobs:
     steps:
 
       - name: "Download HTML documentation artifacts"
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: documentation-html
           path: documentation-html