[C#] Use html entities in javadoc of generated code

Description

Similar to https://github.com/OpenAPITools/openapi-generator/pull/106, we want to apply similar fix to C# client: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/csharp/api.mustache#L33

openapi-generator version

Latest master

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/csharp/api.mustache#L33

Suggest a fix/enhancement

This change should be fairly straightforward. If anyone wants to help on this, please reply to let us know.