Bundle Python runtime built for iOS 15 / macOS 12; CPython 3.12.15/3.13.16/3.14.8

[FeodorFitsner]

Fixes #6940. Targets flet-1.1.0.

Draft: these releases have to be published first, in this order:

1. Release 1.11.0: raise Apple minimums to iOS 15.0 / macOS 12.0 dart-bridge#23 → dart_bridge 1.11.0
2. Bump Pythons to 3.12.15/3.13.16/3.14.8; iOS 15.0 / macOS 12.0 minimums; dart_bridge 1.11.0 python-build#46 → new python-build release (date-keyed)
3. Bump to 6.0.0: iOS 15.0 / macOS 12.0 minimums, CPython 3.12.15/3.13.16/3.14.8 serious-python#256 → serious_python 6.0.0 on pub.dev

Then finish this PR:

• sdk/python/packages/flet-cli/src/flet_cli/utils/python_versions.py: PYTHON_BUILD_RELEASE_DATE = "<python-build date>"
• sdk/python/templates/build/{{cookiecutter.out_dir}}/pubspec.yaml: serious_python: 6.0.0

The pins stay at 20260921 / 5.0.0 for now, because changing them before the releases exist would break flet build in CI.

Why

#6914 moved the build template and client to macOS 12.0 / iOS 15.0 for Xcode 27. The binaries flet build bundles still declared older minimums:

• Python.framework and the per-module Python frameworks: iOS 13.0 / macOS 11.0.
• dart_bridge: iOS 13.0 / macOS 10.15.
• serious_python podspec / SwiftPM: iOS 13.0 / macOS 11.0.

The upstream PRs rebuild all of them for iOS 15.0 / macOS 12.0 and move CPython to the latest patches (3.12.15 / 3.13.16 / 3.14.8). Pyodide stays at 0.27.8 / 0.29.5 / 314.0.7.

Changes so far

• CHANGELOG.md (1.1.0 → Improvements): added an entry for serious_python 6.0.0, the CPython patches and dart_bridge 1.11.0, all built for iOS 15 / macOS 12.
• The minimum-macos-12-ios-15 breaking-change guide now notes that the bundled Python runtime, its native modules and dart_bridge use the same minimums, and links chore: Release serious_python and python_build with minimum iOS 15.0 #6940.
• flet_web/patch_index.py: fixed a stale Pyodide version comment.

Testing (after the pins are set)

• Run flet build macos and flet build ios-simulator on a hello app with a cold ~/.flet/cache. Check that 3.14.8 is resolved and that otool -l on the bundled Python.framework shows minos 12.0 / 15.0.
• uv run pytest sdk/python/packages/flet-cli/tests/test_python_versions.py

Summary by Sourcery

Align the bundled Python tooling and native bridge with the macOS 12 and iOS 15 build targets while refreshing supported runtime versions.

Enhancements:

• Align the bundled Python runtime, native modules, and Dart-Python bridge with the macOS 12 and iOS 15 deployment minimums.
• Update bundled Python versions to CPython 3.12.15, 3.13.16, and 3.14.8 through the updated runtime dependencies.
• Correct the documented supported Pyodide versions and clarify the matching deployment requirements for bundled runtimes.

Documentation:

• Document the updated bundled runtime deployment minimums and link the related issue in the breaking-change guide.
• Add the runtime and dependency updates to the 1.1.0 changelog.

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