[in_app_purchase_android] Introduced new ReplacementMode for Android's billing client

[vongrad]

Introduced new ReplacementMode for Android's billing client and deprecated ProrationMode.

This PR is a follow-up on #6403, where it was decided that we should not replace the ProrationMode with ReplacementMode, but instead only deprecate ProrationMode. The reason for a new PR is also that in_app_purchase_android version 0.3.3 changed internal platform communication to Pigeon, which meant I had to make major changes to my original PR.

List which issues are fixed by this PR. You must list at least one issue.
flutter/flutter#128957

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
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• I [linked to at least one issue that this PR fixes] in the description above.
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• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
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[vongrad]

@bparrishMines I have made this PR in favor of #6403 (comment). Any chance it can be reviewed?

[bparrishMines]

LGTM

@gmackall could you give this a secondary review

[gmackall]

LGTM minus the nits @bparrishMines requested

[vongrad]

I made the requested changes. I have however made a mistake with rebasing the branch with main and pushing, hence all the new required reviewers. I rolled it back, but the extra reviewers remained on the PR, any chance that you can remove them?

[vongrad]

Anything missing from my end?

[vongrad]

@bparrishMines I have made the changes you requested, could you review it?

[bparrishMines]

LGTM