Update BillingClient version to 6.2.1

[vegaro]

Looking at the AndroidManifest.xml I see something new:

        <activity
            android:name="com.android.billingclient.api.ProxyBillingActivityV2"
            android:configChanges="keyboard|keyboardHidden|screenLayout|screenSize|orientation"
            android:exported="false"
            android:theme="@android:style/Theme.Translucent.NoTitleBar" />
    </application>

But after looking at the contents of the ProxyBillingActivityV2 and ProxyBillingActivity I see they've removed some stuff from ProxyBillingActivity related to Alternative Billing, so I think this new activity is being used for alternative billing

Ran some tests, like purchasing and getting products and seems to be working fine.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.37%. Comparing base (4a70f26) to head (8c97012).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1680 +/- ## ========================================= + Coverage 0 83.37% +83.37% ========================================= Files 0 220 +220 Lines 0 7511 +7511 Branches 0 1059 +1059 ========================================= + Hits 0 6262 +6262 - Misses 0 831 +831 - Partials 0 418 +418 ```

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