[FIX] base_gengo: Gengo callback tracebacks

There's two ways Odoo can get job responses from Gengo: the first is
this callback, initiated by Gengo. The second is a cronjob ran by Odoo
which retrieves translated terms. The callback resulted in a traceback
in the logs, because there was a typo in 'base.gengo.translations' (it
was missing an -s at the end). `json.loads` was also being called with
an unexpected extra parameter ('utf-8'), resulting in another traceback.

This PR fixes both of those and I was able to confirm the callback was
able to update translations in Odoo as a result.

closes odoo/odoo#75230

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>