Drop support for iOS 15

[jessesquires]

Drop support for iOS 15. Minimum iOS 16 now required.

Also update Xcode 26 settings.

[Copilot]

Pull request overview

This PR drops support for iOS 15 and establishes iOS 16 as the new minimum requirement for ReactiveCollectionsKit. The changes include updating project configurations, dependency manifests, documentation, and modernizing iOS 16+ API usage.

Key changes:

• Updated minimum iOS deployment target from 15.0 to 16.x across all project files
• Modernized Xcode project files to use object version 90 (from 77)
• Migrated example code to use iOS 16+ collection view layout APIs

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
ReactiveCollectionsKit.xcodeproj/project.pbxproj	Updated Xcode project format to version 90, set iOS deployment target to 16.6, and updated LastUpgradeCheck to 2610
README.md	Updated minimum iOS requirement from 15.0+ to 16.0+
Package.swift	Changed platform requirement from iOS(.v15) to iOS(.v16)
Example/Sources/Grid/GridViewController.swift	Migrated to iOS 16+ API using repeatingSubitem parameter and adjusted item height dimension
Example/ExampleApp.xcodeproj/project.pbxproj	Updated Xcode project format to version 90 and set iOS deployment target to 16.6
CHANGELOG.md	Added entry documenting the iOS 15 support drop

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