iOS: Update minimum iOS version to 13.0

[cbracken]

This updates the Flutter minimum iOS version from 12.0 to 13.0, adds a migrator for existing apps, and updates our own examples, tests, and benchmark apps to 13.0. A follow-up patch will drop iOS 13 @available checks in the embedder.

This is required in order to use Swift in the embedder and not need to bundle the Swift runtime libs in every app that uses Flutter. Swift stable ABI

As of March 2025, usage of iOS is well below 1%, see example public usage data here:
https://telemetrydeck.com/survey/apple/iOS/majorSystemVersions/

This patch makes the following changes:

1. Updates ios_deployment_target from 12.0 to 13.0.
2. Changes templates to IPHONEOS_DEPLOYMENT_TARGET, MinimumOSVersion, and Podfile platform :ios to 12.0.
3. Adds migrator for Podfile part to migrate platform :ios, '11.0' -> platform :ios, '12.0'
4. Compiles with -miphoneos-version-min=12.0
5. Runs the migrator on all example apps and integration tests.
6. Updates examples, tests to iOS 13 deployment target

It also updates verify_exported.dart:

• iOS 13 introduces stricter separation of const and non-const global symbols. Previously, these were declared in the Mach-O __DATA section which may be mapped read-write, but now they're in a dedicated __DATA_CONST section which is mapped read-only. This adds (__DATA_CONST,__const) to the allowlist with the same enforcement on exported symbol naming as before.

See also (ios_deployment_target):

• Set ios_deployment_target to iOS 12.0 buildroot#808
• Set ios_deployment_target to iOS 11.0 buildroot#574

See also (template, migrator):

• Change minimum iOS version from 8.0 to 9.0 #62902
• Migrate iOS app deployment target from 8.0 to 9.0 #85174
• Set template and migrate apps to iOS 11 minimum #101963
• Set template and migrate apps to iOS 12 minimum #140478

Issue: #167735

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.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[stuartmorgan-g]

LGTM