Skip safe atomic swap if update has custom update security policy

[zorgiepoo]

Fixes #2591

Misc Checklist

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

Testing

Also we emit a warning if the app has a NSUpdateSecurityPolicy when checking if the updater is configured correctly.

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

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

Workflows:

• Tested old notarized app (no NSUpdateSecurityPolicy) updating to new notarized app (no NSUpdateSecurityPolicy) (regular workflow)
• Tested old notarized app (setapp-like NSUpdateSecurityPolicy) updating to new notarized app (no NSUpdateSecurityPolicy) -- atomic swap works without prompt
• Tested old notarized app (no NSUpdateSecurityPolicy) updating to new notarized app (setapp-like NSUpdateSecurityPolicy) -- no atomic swap
• Tested old notarized app (setapp-like NSUpdateSecurityPolicy) updating to new notarized app (setapp-like NSUpdateSecurityPolicy) -- no atomic swap
• Tested warnings are logged whenever app adopts NSUpdateSecurityPolicy

macOS version tested: 14.5 (23F79)