ci: update Swift test matrices across platforms

[ctreffs]

Description

Updates the CI test workflow matrices across macOS, iOS, Linux, and Windows to test Swift 6.1 and 6.3 (except Linux which tests 6.1 and 6.2), and updates the documentation build workflow to use the native system toolchain.

Summary of changes:

• .github/workflows/ci-macos.yml: Configured matrix for Swift 6.1 on macos-14 runner and Swift 6.3 on macos-latest runner, skipping setup-swift for 6.3.
• .github/workflows/ci-ios.yml: Updated matrix to Swift 6.1 and 6.3, skipping setup-swift for 6.3.
• .github/workflows/ci-linux.yml: Updated matrix to Swift 6.1 and 6.2.
• .github/workflows/ci-windows.yml: Updated matrix to Swift 6.1 and 6.3.
• .github/workflows/docs.yml: Removed redundant setup-swift step to build documentation using native system Xcode on macos-latest.

Rationale

Resolves toolchain and SDK mismatch failures on macos-latest runners by utilizing native system Xcode toolchains for Swift 6.3 and matching runner environments (macos-14) for Swift 6.1. Ensures all platforms maintain target Swift compatibility testing.

Detailed Design

Adjusted workflow matrix configurations, runner assignments, and toolchain setup conditions in GitHub Actions workflow files.

Testing

Verified all 10 CI jobs across macOS, iOS, Linux, Windows, and Documentation pass on GitHub Actions for PR 50.

Source Impact

No impact on package source code; configuration changes strictly apply to CI workflow execution.

Checklist

• I've read the Contribution Guidelines
• I've followed the coding style of the rest of the project.
• I've verified that my change does not break any existing tests or introduce unexpected benchmark regressions.
• I've updated the documentation (if appropriate).