Version 4: Make v4 the stable release

### Summary
The final step of v4 preparation / migration: making v4 the stable release and updating the v4 website URL.

### 💬 Description
The ticket is basically to officially move everything over to v4.

Actions:
- Update the v4 release workflow to use the `latest` tag
- Update the v3 release workflow to no longer use the `latest` tag, e.g. we could change it to use `v3-latest`, similar to [v2](https://github.com/mi6/ic-ui-kit/blob/8340cf65d30bdcce56713f3d28c8b72373136a74/.github/workflows/ic-ui-kit-v2-release-merged.yml#L174)
- Update the v4 main branch website deployment to be at our official / normal URL (https://design.sis.gov.uk/) and change the v3 website URL

**Note:** there are a few details which need discussing before work starts on this ticket - see comment below

Review the v3 and v4 release workflows under .github/workflows, using the referenced v2 workflow as a comparison, and read the pending discussion before starting. Update the release tags and website deployment URLs so v4 uses the official URL and latest tag while v3 uses a separate tag and URL; verify both release workflows and deployments.

Written by the indexing model from the issue text.