feat(template): use latest and update documentation

Per Docker Images Versions[1] and Latest version by default[2] update the cypress image to use
latest.

It's unclear from the history why Cypress is using a version and not latest. Possibly the "tool"
exception[3]. However, this appears to be directed at implementation users not TBC directly.
Additionally, the core team has never updated this to use latest, but seeing the number of trivy
vulnerabilities in the 13.x line, an update to use latest seemed prudent.

1. https://to-be-continuous.gitlab.io/doc/usage/#docker-images-versions
2. https://to-be-continuous.gitlab.io/doc/dev/architecture/#latest-version-by-default
3. https://to-be-continuous.gitlab.io/doc/dev/architecture/#tool-versions-must-be-selected-with-image-tags

Fixes: cypress#14