build(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] <support@github.com>

Author: dependabot[bot]

.github/workflows/gubbins-test.yml

@@ -226,7 +226,7 @@ jobs:
       # WARNING: In your CI/CD pipeline, you should remove the following steps  #
       # ------------------------------------------------------------------------#
       - name: Download gubbins-web
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: gubbins-web
           path: /tmp/gubbins-web
@@ -259,7 +259,7 @@ jobs:
     # WARNING: In your CI/CD pipeline, you should remove the following steps  #
     # ------------------------------------------------------------------------#
     - name: Download gubbins
-      uses: actions/download-artifact@v5
+      uses: actions/download-artifact@v6
       with:
         name: gubbins
         path: /tmp/gubbins
@@ -274,7 +274,7 @@ jobs:
         git clone https://github.com/DIRACGrid/diracx.git
 
     - name: Download gubbins:services image
-      uses: actions/download-artifact@v5
+      uses: actions/download-artifact@v6
       with:
         name: gubbins-services-img
         path: /tmp/
@@ -283,7 +283,7 @@ jobs:
       run: docker load --input /tmp/gubbins_services_image.tar
 
     - name: Download gubbins-web
-      uses: actions/download-artifact@v5
+      uses: actions/download-artifact@v6
       with:
         name: gubbins-web
         path: /tmp/gubbins-web