Integrations: migrate generic integrations token to be stored in the `secret` field

[stsewd]

What's the problem this feature will solve?

The secret token of the generic integrations is being saved in provider data attribute

https://github.com/readthedocs/readthedocs.org/blob/4522650bd7bb92b046d9a982b2a5f74fe6502332/readthedocs/integrations/models.py#L377-L387

Which is json field, we have a dedicated attribute for such thing secret.

https://github.com/readthedocs/readthedocs.org/blob/4522650bd7bb92b046d9a982b2a5f74fe6502332/readthedocs/integrations/models.py#L302-L307

Describe the solution you'd like

Migrate the secret token from provider_data into secret for all generic integrations.