Remove all references to `Secret` and `ApiKey`

[tomasr78]

• Remove all references to Secret and ApiKey, they are deprecated, for authentication, Api Token is used.
• Use placeholder api_token

Example

• Update get credentials url to https://www.convertapi.com/a/authentication

Example

[paulius-petkus]

@tomasr78 I can see commits it seems done. Maybe we can close it?