[BUG][C#] Regex Strings both Escaped and Literal

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When using a patterned string with escaped characters, the generated regex in the C# model both escapes the backslashes and makes the string literal. It should be one or the other. For example, pattern: '/^[a-z0-9]([a-z0-9\-\_ ]*[a-z0-9])?$/i' results in new Regex(@"^[a-z0-9]([a-z0-9\\-\\_ ]*[a-z0-9])?$", RegexOptions.CultureInvariant | RegexOptions.IgnoreCase).

openapi-generator version

5.1.0

OpenAPI declaration file content or url

swagger: "2.0"
info:
  title: Example
  version: 1.0.0
paths:
  /fake:
    get:
      responses:
        "400":
          description: fake
definitions:
  SomeType:
    type: object
    required:
      - name
    properties:
      name:
        type: string
        pattern: '/^[a-z0-9]([a-z0-9\-\_ ]*[a-z0-9])?$/i'

Generation Details

docker run --rm -v ${PWD}:/local openapitools/openapi-generator-cli generate `
    --global-property modelTests=false,modelDocs=false,apiTests=false,apiDocs=false `
    -i /local/$swaggerSpecFile `
    -g csharp-netcore `
    -o /local `
    -c /local/openapi-generator-config.json

{
  "packageName": "SomeProject",
  "targetFramework": "netstandard2.0",
  "optionalAssemblyInfo": false,
  "optionalProjectFile": false
}

Steps to reproduce

1. Create swagger.yml with the above contents in new folder.
2. Create powershell file with the above contents in same folder.
3. Create openapi-generator-config.json with above contents in same folder.
4. Install docker if it isn't, making sure to add it to path.
5. Run docker pull openapitools/openapi-generator-cli in a terminal.
6. Run the created powershell file in the same folder.
7. Examine generated src/SomeProject/Model/SomeType and note regex associated with Name.

Related issues/PRs

None that I was able to find.

Suggest a fix

Either make the string not literal (remove the @ from @"{REGEX}") or remove the escaping backslashes.

Reproduce the issue using the swagger.yml declaration, the PowerShell generation script, and openapi-generator-config.json. Inspect the generated src/SomeProject/Model/SomeType model and trace the C# generator template or entry point responsible for the Name regex. Done means the generated pattern uses compatible escaping and literal-string handling without changing the intended regex.