Update default schema and SepaDD to be compliant with current standart by default

[philipp-horstenkamp]

Please update the default values from:
Python

from sepaxml import SepaDD

SepaDD(config, schema="pain.008.001.02", clean=True)

to:

from sepaxml import SepaDD

SepaDD(config, schema="pain.008.001.08", clean=False)

This will ensure compliance with current mandatory standards.

I would also suggest moving the text_unidecode dependency to an extra, as it won't be needed by everyone.

I am happy to submit a PR for this. However, since I already have an open PR, I wanted to check first if there is a good chance it will be reviewed and merged before I open a new one.