[in_app_purchase_storekit] Use _$SKPaymentDiscountWrapperToJson

[ValentinVignal]

Part of flutter/flutter#183937

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

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[gemini-code-assist]

Code Review

This pull request updates the toMap method in SKPaymentDiscountWrapper to use the generated _$SKPaymentDiscountWrapperToJson(this) function instead of manually constructing the map. I have no feedback to provide.

[ValentinVignal]

I'd assume this would rely on existing tests to pass and doesn't require a new test to be written?
Does it need a new entry

## NEXT

- Fixes lint issues.

in the changelog? Or is it better to ignore it?

[stuartmorgan-g]

[x] I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under1.

I'm not a primary reviewer for this package, but as a general note: please don't check items in the checklist if you haven't done them. There are no version or changelogs in this PR, and no reference is made in the PR description or comments to any of the documented exceptions.

[ValentinVignal]

@stuartmorgan-g , sure noted, this comment was addressing (or raising) the CHANGELOG matter.

I will wait for the primary reviewers of this package to let me know what they think about it. Thank you for letting me know

[stuartmorgan-g]

this comment was addressing (or raising) the CHANGELOG matter.

That comments asks if this PR should follow our documented policy or not, which isn't what the checklist item says. It also doesn't address the missing version change at all.

If you believe this PR should be exempted from standard policy it's fine to ask (although generally such a request should come with an explanation of why you believe it should be exempt), but please don't also check the box in the checklist that says that you did follow the policy.