ci: migrate Apple jobs to the xcode-27 runner

[SelaseKay]

Description

Move iOS and macOS CI off macos-15 / Xcode 26.2 onto GitHub's public-preview xcode-27 image so we actually build and test against Xcode 27, iOS 27, and macOS 27.

That image does not include Xcode_26.2.app or an iPhone 16 simulator, so the old xcode-select pin is removed (the image default is already Xcode 27) and simulator jobs use iPhone 17. Xcode compile and Pods cache keys are bumped so we do not restore Xcode 26 artifacts onto 27.

Format, FDC web/wasm, and pipeline web stay on macos-latest.

The xcode-27 image is still in public preview and currently ships Xcode 27.0 beta. First runs will be a cold cache.

Related Issues

N/A

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

Test plan

• Confirm iOS e2e jobs pick up xcode-27 and boot an iPhone 17 simulator
• Confirm macOS e2e and Swift integration jobs compile on the image-default Xcode 27
• Confirm smoke, FDC iOS, and Firestore pipeline iOS still pass (cold cache expected on first run)
• Confirm format / web jobs still run on macos-latest

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