Bump actions/download-artifact from 5.0.0 to 6.0.0 (#949)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@634f93c...018cc2c)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: 6.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Matt Jarvis <[email protected]>

Author: dependabot[bot]

.github/workflows/deploy-backend.yml

@@ -105,7 +105,7 @@ jobs:
           terraform_version: "1.12.2"
 
       - name: Retrieve Terraform Plan
-        uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0
+        uses: actions/download-artifact@018cc2cf5baa6db3ef3c5f8a56943fffe632ef53
         with:
           name: ${{ env.ENVIRONMENT }}-${{ env.SUB_ENVIRONMENT }}-tfplan
           path: infrastructure/instance