Update com.android.billingclient:billing library to 7+ version

[savelyevdm]

Hi!
Google recently sent us a notice stating that our app, which includes myTracker, does not comply with its policies - specifically this one:

Reminder: By Aug 31, 2025, all new apps and updates to existing apps must use Billing Library version 7 or newer.

I inspected our Android dependency tree and found that it’s your library (v. 3.4.2) that’s blocking the use of version 7 and above:

|    |    +--- com.my.tracker:mytracker-sdk:3.4.2
|    |    |    +--- com.android.billingclient:billing:[5.0.0, 7.0.0) -> 6.2.1

Please update your SDK so that we can continue using it.