[RFC] updating the minimum SDK version to API 21

[tobrun]

This issue tracks the conversation on updating the library minimum SDK version to API 21. The requirement is not something that originates from Mapbox but is being passed down by the OkHttp dependency. We have tried with using the LTS version of OkHttp 3.12.12 but due to the problems laid out below. We feel, we are forced to migrate to OkHttp 4.x which will result in bumping the mininum SDK version of this library to 21.

OkHttp 3.12.x series is compatible with Android 19 (our current min. SDK version), when users use 3.14 of the library, which can come from other transitive dependencies as retrofit. It makes our SDK unusable on API levels lower as 21 without the developer knowing. This is a bad UX and out of our control to fix.

Note that you can force okhttp 3.12.12 in this scenario but this is nothing from the SDK side we can enforce:

configurations.all {
    resolutionStrategy {
        force "com.squareup.okhttp3:okhttp:3.12.12"
    }
}

We are proposing now to update OkHttp to the 4.x series and increasing our minimum SDK version to 21, this not only removes the security concerns from Square but also makes the SDK itself more future proof. SDKs when compared to apps have a longer turn around time to be updated and can stay longer active in production. Considering OkHttp 3.12 is out-of-live at the end of the year. This seems like the appropriate time for us to update.

refs ##55

[tobrun]

with beta.16, okhttp 4.x landed!