HaskellServantCodegen and HaskellYesodServerCodegen modify specialCharReplacements by replacing its keys: backslash (\) with \\ and " with \".

It seems that those replacements were for using the keys in string literals in the specialChars table in the generated code.
I.e. we want to have ("\\", "Back_Slash") instead of ("\", "Back_Slash") and ("\"", "Double_Quote") instead of (""", "Double_Quote") in the table below.

However, modifying the keys causes the substitution of those characters in field names not to work, making generated code syntactically invalid. Suppose we have the following spec:

openapi: 3.0.0
info:
  title: Test
  description: Test API
  version: 1.0.0
servers:
  - url: https://api.example.com/
    description: the server

paths:
  /hello:
    get:
      operationId: hello
      summary: "summary"
      description: "description."
      responses:
        "200":
          description: foo
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/HelloResponse'

components:
  schemas:
    HelloResponse:
      type: object
      properties:
        "\"":
          type: string
          description: double quote
        "\\":
          type: string
          description: backslash
      required:
        - "\""
        - "\\"
      description: description

Those generators generate the following data type definition, but helloResponse" and helloResponse\ are invalid identifiers in Haskell.

-- | description
data HelloResponse = HelloResponse
  { helloResponse" :: Text -- ^ double quote
  , helloResponse\ :: Text -- ^ backslash
  } deriving (Show, Eq, Generic, Data)

Since the specialChars table has already been removed in #16232, we can safely stop modifying the specialCharReplacements, thereby generated code for the above example become the following syntactically correct code.

-- | description
data HelloResponse = HelloResponse
  { helloResponseDoubleQuote :: Text -- ^ double quote
  , helloResponseBackSlash :: Text -- ^ backslash
  } deriving (Show, Eq, Generic, Data)

I believe that this change maintains backward compatibility, as it only fixes cases where it has not been working (when field names contain backslashes or double quotes).

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