Renamed Xarxa Greta Fiber from 1Gb to 600Mb

CHANGELOG:

### CHANGED
- [#625](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/625) Renamed Xarxa Greta Fiber from 1Gb to 600Mb

Related card: https://trello.com/c/gUBg8BbN/956-canvi-de-nom-dels-productes-greta-a-odoo

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