fix: SPM manifest compilation on Swift 6.1

[WhiteHyun]

📜 Description

Remove the encoding: argument from String(cString:) in Package@swift-6.1.swift so the manifest compiles on Swift 6.1 / Xcode 16+.

💡 Motivation and Context

String(cString:encoding:) is a Foundation extension on String and is not available in the manifest context. On Swift 6.1 / Xcode 16+, manifest compilation fails with:

/Package@swift-6.1.swift:72:51: error: extra argument 'encoding' in call
/Package@swift-6.1.swift:72:51: error: cannot infer contextual base in reference to member 'utf8'

This breaks swift package resolve for downstream projects consuming affected 8.x versions on Xcode 16+.

The stdlib String(cString:) already decodes the C string as UTF-8, so behavior is preserved.

Fixes #7797

💚 How did you test it?

• swift package dump-package now succeeds on Xcode 16 / Swift 6.1.
• End-to-end dependency resolution was verified on a consumer project pinning sentry-cocoa in the 8.x range.

No unit tests added — this is a manifest-only change, and the current
test/CI setup does not include a dedicated manifest-compilation check.

📝 Checklist

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

[sdk-maintainer-bot]

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[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

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Bug Fixes 🐛

• SPM manifest compilation on Swift 6.1 by WhiteHyun in #7800

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