fix: SPM manifest compilation on Swift 6.1

[itaybre]

This PR is a clone of #7800 because it failed due to GH permissions, it should pass when created by a Collaborator

📜 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.

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1227.35 ms	1261.76 ms	34.41 ms
Size	24.15 KiB	1005.14 KiB	980.99 KiB

Baseline results on branch: v8.x

Startup times

Revision	Plain	With Sentry	Diff
b79b552	1210.26 ms	1232.54 ms	22.28 ms
e537c90	1226.22 ms	1256.64 ms	30.41 ms
b66be9b	1218.22 ms	1244.19 ms	25.96 ms
e5256c6	1216.34 ms	1248.29 ms	31.95 ms
33c9619	1220.80 ms	1240.02 ms	19.22 ms
5e3fb04	1239.84 ms	1267.39 ms	27.55 ms
e5256c6	1240.96 ms	1273.33 ms	32.38 ms
3af1ae9	1225.60 ms	1252.65 ms	27.05 ms
33c9619	1233.02 ms	1252.60 ms	19.58 ms
b79b552	1216.04 ms	1241.27 ms	25.22 ms

App size

Revision	Plain	With Sentry	Diff
b79b552	23.75 KiB	992.25 KiB	968.50 KiB
e537c90	23.75 KiB	992.03 KiB	968.28 KiB
b66be9b	23.75 KiB	996.03 KiB	972.28 KiB
e5256c6	24.14 KiB	1004.95 KiB	980.80 KiB
33c9619	24.14 KiB	1001.22 KiB	977.08 KiB
5e3fb04	23.74 KiB	981.30 KiB	957.56 KiB
e5256c6	24.14 KiB	1005.14 KiB	980.99 KiB
3af1ae9	23.74 KiB	981.29 KiB	957.55 KiB
33c9619	24.14 KiB	1001.34 KiB	977.19 KiB
b79b552	23.75 KiB	992.16 KiB	968.41 KiB