Ensure date-based versions can be deployed to the root

[jsanz]

Part of elastic/kibana#157094

maps.elastic.co will point to the latest release of the EMS Landing Page. We will parameterize that with date-based versioning. This is already supported since the branch to put on the root is parametrized, and we allow both semver and date-based branches for deployment, but we need to ensure that this process works as expected and eventually automate the release of the root branch without needing to manually set it up.