fix(insights): use latest appId and apiKey

[aymeric-giraudet]

Summary

FX-3372

Fixes #6623

Result

We should use the latest appId and apiKey as the latter can be rotated

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[Haroenv]

This makes sense, do we still use the initial appid,apikey then? can we change it so it's only retrieved in sendEvent itself?

[aymeric-giraudet]

@Haroenv Retrieving it when the middleware starts is only for old versions of search insights now, for this users will have to call init if they change their apiKey. For modern versions, we retrieve it each time in sendEvent, init wouldn't work anyway as we use the new signature to force headers.