[C#] Instantiation type is ignored

Description

When using --type-mapping defining interfaces for list and adding --instantiation-types to provide a concrete type when required, whenever the contract defines an array type, the template output defined the array type inheriting from the interface (type-mappings) and not the concrete type (instantion-types)

The result in the output is for modules/swagger-codegen/src/test/resources/3_0_0/petstore-with-fake-endpoints-models-for-testing.yaml for one of the classes in this example the output is:
public partial class AllPetsResponse : IReadOnlyList<OneOfAllPetsResponseItems> ...
instead of
public partial class AllPetsResponse : List<OneOfAllPetsResponseItems>

Swagger-codegen version

3.0.46, but this issue was present for a long time. I'm not sure if we can consider this an issue or a feature request.

Swagger declaration file content or url

Use the sample in the source code modules/swagger-codegen/src/test/resources/3_0_0/petstore-with-fake-endpoints-models-for-testing.yaml

Command line used for generation

The interesting part of the command to reproduce this is:

--type-mappings array=IReadOnlyList --instantiation-types array=List

The full command is

generate -i modules/swagger-codegen/src/test/resources/3_0_0/petstore-with-fake-endpoints-models-for-testing.yaml -l csharp -o samples/client/petstore/csharp/SwaggerClientNetStandard --additional-properties packageGuid={321C8C3F-0156-40C1-AE42-D59761FB9B6C} -c ./bin/csharp-petstore-net-standard.json --type-mappings array=IReadOnlyList --instantiation-types array=List

NOTE this is the command for generating the sources for csharp netstandard from csharp-petstore-net-standard.sh with the interesting part appended to it

Steps to reproduce

1. Run csharp-petstore-net-standard.sh

csharp-petstore-net-standard.sh --type-mappings array=IReadOnlyList --instantiation-types array=List

2. Check the output and you will notice the behavior described

Related issues/PRs

Suggest a fix/enhancement

For the base of classes for C#, the values from --instantiation-types should be used instead of the values from --type-mappings

Run csharp-petstore-net-standard.sh with --type-mappings array=IReadOnlyList --instantiation-types array=List using petstore-with-fake-endpoints-models-for-testing.yaml. Trace the C# generation path and templates that determine the base type for AllPetsResponse. Done means the generated class uses List while other type mappings remain correct.