Make CLI state pruning optional again (#13017)

* Make CLI state pruning optional again

The state pruning setting is stored in the database when it is created. In later runs it is fine to
drop the `--state-pruning` CLI argument as the setting is stored in the database. The state db will
only return an error if the stored state pruning doesn't match the state pruning given via CLI.

Recently we improved the state pruning CLI handling and accidentally made the state pruning value
always present (as we set some default value for the clap). If we could find out if a user has
passed a value or the default value was taken, we could keep the default value in the CLI interface,
but clap isn't supporting this right now. So, we need to go back and make `state_pruning` an
optional with the default written into the docs.

It also adds a test to ensure that we don't break this behavior again.

* More docs