[BUG] [csharp-netcore] Obsolete method warning

[etherealjoy]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
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Description

The RestSharp RestClient throws a warning about deprecated/obsolete
csharp-netcore tfm 2.0

1>Client\ApiClient.cs(385,17,385,133): warning CS0618: 'RestClient.AddHandler
(IDeserializer, params string[])' is obsolete: 
'Use the overload that accepts a factory delegate'
1>Client\ApiClient.cs(390,17,390,118): warning CS0618: 'RestClient.AddHandler(
IDeserializer, params string[])' is obsolete: 
'Use the overload that accepts a factory delegate'
1>Client\ApiClient.cs(393,13,393,98): warning CS0618: 'RestClient.AddHandler(
IDeserializer, params string[])' is obsolete: 
'Use the overload that accepts a factory delegate'

openapi-generator version

master

OpenAPI declaration file content or url

no special yaml needed. Even default petstore cause same issue.

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