feat: upgrade to casbin v2

[msfreed]

Fix: #9

• Dropped support for netcoreapp3.1 and net5.0
• Dropped Newtonsoft in favor of System.Text.Json
• Added support for net8.0
• Upgraded package references
• Simplified RedisWatcher and removed the generic version

[casbin-bot]

@sagilio @AsakusaRinne please review

[CLAassistant]

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[hsluoyz]

@msfreed plz fix CI:

[msfreed]

How do you guys feel about only supporting MS supported target frameworks (e.g. net6, net7, net8, netstandard2.1)?

[hsluoyz]

How do you guys feel about only supporting MS supported target frameworks (e.g. net6, net7, net8, netstandard2.1)?

@msfreed is it easy to keep net5 support? If it's not, it's also OK to upgrade

[hsluoyz]

@msfreed plz sign CLA

[github-actions]

🎉 This PR is included in version 1.2.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[msfreed]

I've signed a few times, not sure why it isn't recognized yet