Expose maximum-versions option to preserve in feed per branch

[zorgiepoo]

Also let a zero or negative value mean that all versions should be preserved.

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

Only bug fixes to regressions or security fixes are being backported to the 1.x (master) branch now. If you believe your change is significant enough to backport, please also create a separate pull request against the master branch.

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 generate_appcast using this option and passing different values to it, including 0.

macOS version tested: 12.5.1 (21G83)