Remove the `GetApiInstance` function

[toddbaert]

Remove the GetApiInstance function. Exposing the fact that the API is a singleton to users does not make sense. That is an internal implementation detail that users do not need to be aware about. It is also very clunky to have users do openfeature.GetApiInstance().GetClient(). Instead, have the user do openfeature.NewClient().

Remove openfeature.GetApiInstance().GetClient() and only export openfeature.NewClient()

[jblacker]

Looked like a quick win so I went ahead and handled the non-breaking portion of this ticket.

[jblacker]

I think that this needs to be re-opened. My change is only part 1 that handles all of the non-breaking updates. The breaking updates suggested in this issue have not yet been completed.

[sahidvelji]

I'll reopen it. Technically the removal of deprecated identifiers is covered by #410, but I think it's fine to track this separately.