[various] Scrub pre-iOS-13 code

[stuartmorgan-g]

For all packages that had conditionals to support iOS <13:

• Update the min Flutter version to 3.35, which requires iOS 13
• Remove the conditional logic
• Update all the project metadata to require iOS 13+ (and macOS 10.15+ where applicable)

There is also some minor cleanup in in_app_purchase_storekit; just removing conditionals would have left cases where we were unnecessarily constructing dictionaries dynamically, so I folded them into the initial dictionary construction, and while doing that I noticed some cases where we were doing other unnecessary dynamic construction (maybe due to previous OS conditionals?) so I updated those as well. And then I noticed some inconsistency in use of WithCapacity: and updated those cases to be consistent. Lastly, I fixed a few stale comments that didn't reflect the current state of the code.

Includes autoformat changes, since updating the min Dart SDK opts into some formatter changes.

Fixes flutter/flutter#167735

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

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[gemini-code-assist]

Code Review

This pull request effectively removes support for iOS versions older than 13.0 across several packages, which simplifies the codebase by removing conditional logic and updating deployment targets. The changes are well-executed and include valuable refactoring in in_app_purchase_storekit to improve code clarity and performance by using dictionary literals and pre-allocating array capacities. I've found one minor inconsistency in the test stubs related to array initialization that could be addressed for better consistency with the rest of the changes.

[gemini-code-assist]

For consistency with the other changes in this PR, it would be better to initialize this mutable array with a specific capacity, since the size is known. You've done this in FIAObjectTranslator.m, and applying the same pattern here would be a nice touch.

Suggested change

	NSMutableArray *discounts = [[NSMutableArray alloc] init];
	NSMutableArray *discounts = [NSMutableArray arrayWithCapacity:[map[@"discounts"] count]];

[stuartmorgan-g]

This is actually ready for review now; apologies for not marking it as a draft when I first posted it.