Use LaTeX's rollback support for backwards compatibility

[logological]

Currently heria defines a user-facing \templateversion command that can be used to specify which version of the Horizon Europe template to use; if this command is not used, then the class uses the latest version of the template.

However, LaTeX already provides a set of standard commands for this purpose: see A rollback concept for packages and classes. heria should be revised to use these commands.