Don't allow removal of signing keys more strictly

[zorgiepoo]

• Don't allow removal of (Ed)DSA keys for pre-validated updates (delta updates, .aar updates)
• Don't allow removal of code signing identity in new update (at minimum, an adhoc signature can be used)

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

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)

• Tested updating regular EdDSA + code signed app
• Tested updating EdDSA app that drops code signing (fails)
• Tested updating regular EdDSA + code signed app with delta update
• Tested updating EdDSA app that drops code signing with delta update (fails)
• Tested updating EdDSA app that drops EdDSA key with delta update (fails)

macOS version tested: 15.0.1 (24A348)