Bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...v6)

---
updated-dependencies:
- dependency-name: actions/upload-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/build-and-test.yml

@@ -37,7 +37,7 @@ jobs:
       - name: Build installer
         run: pixi run build-installer
       - name: Upload installer
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: installer-${{ matrix.target_arch }}
           path: DIRACOS-*.sh
@@ -61,7 +61,7 @@ jobs:
           source diracos/diracosrc
           conda env export --file environment.yaml --prefix $DIRACOS
       - name: Upload environment.yaml
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: environment-yaml-${{ matrix.target_arch }}
           path: |
@@ -90,7 +90,7 @@ jobs:
             > release-notes.md
           cat release-notes.md
       - name: Upload release-notes.md
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: release-notes
           path: |