Update system benchmark to use `totEvents` and drop deprecated `size` field from `package-spec`

[paolafrancesca]

In package-spec, corpora.generator.size field for the system benchmark scenario was deprecated in favor of corpora.generator.totEvents (see source)

We should update the code defining the yaml schema to use the new field and change in current artifacts the field from size to totEvents and set the relevant equivalent values

Contextually to this change we should update to latest version of corpora generator