Disable sandboxing for downloader XPC service by default

[zorgiepoo]

This disables sandboxing by default for the downloader and status/installer XPC services. If developers want to sandbox these services, they must use a custom bundle ID (XPC_SERVICE_BUNDLE_ID_PREFIX) for their app and enable the sandbox while building Sparkle from source themselves.

Fixes #2507

Misc Checklist

• [x] My change requires a documentation update on Sparkle's website repository
• [ ] My change requires changes to generate_appcast, generate_keys, or sign_update

Testing

I tested and verified my change by using one or multiple of these methods:

• [x] Sparkle Test App
• [ ] Unit Tests
• [ ] My own app
• [ ] Other (please specify)

Tested enabling downloader XPC service in Sparkle test app. Tested making distribution with make release.

macOS version tested: 14.4 Beta (23E5196e)