Fix visibility for enum in C# generator

[ferenc-a]

The visibility of enums in the C# generator is wrong, and results in build errors with nonPublicApi=true.

Here are sanitized build error logs before the fix:

Client\Model\DeviceTypeEnum.cs(54,38): error CS0050: Inconsistent accessibility: return type 'DeviceTypeEnum' is less accessible than method 'DeviceTypeEnumValueConverter.FromString(string)' 
Client\Model\DeviceTypeEnum.cs(70,39): error CS0050: Inconsistent accessibility: return type 'DeviceTypeEnum?' is less accessible than method 'DeviceTypeEnumValueConverter.FromStringOrDefault(string)' 
Client\Model\DeviceTypeEnum.cs(87,30): error CS0051: Inconsistent accessibility: parameter type 'DeviceTypeEnum' is less accessible than method 'DeviceTypeEnumValueConverter.ToJsonValue(DeviceTypeEnum)' 
Client\Model\DeviceTypeEnum.cs(103,18): error CS0060: Inconsistent accessibility: base class 'JsonConverter<DeviceTypeEnum>' is less accessible than class 'DeviceTypeEnumJsonConverter' 
Client\Model\DeviceTypeEnum.cs(112,40): error CS0050: Inconsistent accessibility: return type 'DeviceTypeEnum' is less accessible than method 'DeviceTypeEnumJsonConverter.Read(ref Utf8JsonReader, Type, JsonSerializerOptions)'   
Client\Model\DeviceTypeEnum.cs(132,30): error CS0051: Inconsistent accessibility: parameter type 'DeviceTypeEnum' is less accessible than method 'DeviceTypeEnumJsonConverter.Write(Utf8JsonWriter, DeviceTypeEnum, JsonSerializerOptions)' 
Client\Model\DeviceTypeEnum.cs(141,18): error CS0060: Inconsistent accessibility: base class 'JsonConverter<DeviceTypeEnum?>' is less accessible than class 'DeviceTypeEnumNullableJsonConverter' 
Client\Model\DeviceTypeEnum.cs(150,41): error CS0050: Inconsistent accessibility: return type 'DeviceTypeEnum?' is less accessible than method 'DeviceTypeEnumNullableJsonConverter.Read(ref Utf8JsonReader, Type, JsonSerializerOptions)' 
Client\Model\DeviceTypeEnum.cs(170,30): error CS0051: Inconsistent accessibility: parameter type 'DeviceTypeEnum?' is less accessible than method 'DeviceTypeEnumNullableJsonConverter.Write(Utf8JsonWriter, DeviceTypeEnum?, JsonSerializerOptions)' 

A slightly related issue I found that seems resolved now: #11357.

PR checklist

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  ./mvnw clean package || exit
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  (For Windows users, please run the script in WSL)
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  • @mandrean, @shibayan, @Blackclaws, @lucamazzanti, @iBicha - sorry if I should not have pinged all of you

[wing328]

thanks for the pr

let's give it a try

i'll add a test later

[wing328]

FYI. Merged #21996 to add a test for nonPublicApi option