SDK-3468 Minimal compatibility with BC7

[JayShortway]

Note: target branch is equal to main (at the time of writing).

This adds the minimum required changes to be able to compile with BC7. Tests are also expected to work. (They do locally.)

Commits are pretty isolated, so you could review per commit, to see fewer files at a time.

Breaking changes

• Bumps the version to 8.0.0-SNAPSHOT.
• Bumps the minSdkVersion to 21 (Android 5.0).
• UpgradeInfo now takes a ReplacementMode instead of the now-removed ProrationMode.
• Removed the deprecated GoogleProrationMode, as it was using the now-removed ProrationMode.
• Removes deprecated StoreTransaction constructor taking a GoogleProrationMode.
• Removes PurchaseParams.Builder.googleProrationMode().

Internal changes

• Uses Billing Library 7.0.0.
• Added GoogleReplacementModeAPI.java
• Changed a body parameter of postReceiptData():
  • name: proration_mode -> replacement_mode.
  • value: GoogleProrationMode.name -> GoogleReplacementMode.name

[RevenueCat-Danger-Bot]

	1 Message
:book:	Size increase: 1.45 KB

Generated by :no_entry_sign: Danger

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 83.11%. Comparing base (33a4377) to head (e0849d1).

Files	Patch %	Lines
.../kotlin/com/revenuecat/purchases/common/Backend.kt	50.00%	3 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## sdk-3465-support-bc7 #1707 +/- ## ======================================================== + Coverage 83.07% 83.11% +0.04% ======================================================== Files 222 221 -1 Lines 7581 7547 -34 Branches 1073 1066 -7 ======================================================== - Hits 6298 6273 -25 + Misses 857 850 -7 + Partials 426 424 -2 ```

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