Remove legacy XcResult support for Xcode versions older than 16

[priitlatt]

Motivated by #503 (comment).

Since PR #431 (released in 0.54.0) we have kept two XcResult to JUnit converters and picked one at runtime by the selected Xcode version. Xcode versions older than 16 no longer need support, so this PR removes the legacy converter.

Changes

• Remove LegacyXcResultConverter, the codemagic.models.xctests.xcresult.legacy_xcresult module (ActionsInvocationRecord etc.) and its tests and mocks.
• Remove the XcResultTool methods is_legacy, get_bundle and get_object.
• Merge Xcode16XcResultConverter into XcResultConverter, which is no longer an abstract base class.

Effect

• The xcode-project actions run-tests, test-summary and junit-test-results need Xcode 16+ to parse test results. With older Xcode versions they fail with Parsing test results failed.
• ⚠️ The Python API changes are backwards incompatible.

Updated actions

• xcode-project run-tests
• xcode-project test-summary
• xcode-project junit-test-results

[artemii-yanushevskyi]

Changes look solid.

Consider fail early with clear "Xcode 16+ required". Xcode < 16 no longer has a clear capability gate. Processing could fail with a generic parsing error.