Use generic Enum.IsDefined when Unity supports .NET 5

[elringus]

This can be slow due to boxing:

https://github.com/Elringus/UnityRawInput/blob/c4cada025db85674f584c0330186e8ab3db01605/Assets/UnityRawInput/Runtime/HookArgs.cs#L25-L35

.NET 5+ has generic overload which doesn't involve boxing. We have to use it when Unity allows.

ref: https://stackoverflow.com/questions/13615/validate-enum-values/55028274#55028274