[ci] refactor: Cleanup tooling for handling arch build output <= Flutter v3.13.x

[Gustl22]

There remained some code, which was left in to support build output for Flutter <= v3.13.x.
As the minimum supported Flutter version is 3.35.x as of

packages/.ci/targets/repo_checks.yaml

Line 31 in eab1265

- "--min-min-flutter-version=3.35.0"

we can remove it now.

Contributes to flutter/flutter#129807

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[gemini-code-assist]

Code Review

This pull request updates the Pigeon test runner and the repository's native test tool to utilize dart:ffi's Abi for architecture detection, adding support for Linux RISC-V and ARM64. It also refactors CMakeProject to require an architecture and removes legacy compatibility logic for older Flutter versions. Feedback was provided regarding the architecture detection logic in native_test_command.dart, suggesting an explicit mapping to avoid fragile assumptions when handling multiple architectures.

[Gustl22]

I suspect its

test-exempt: Remove code without adding functionality.