[BUG][C++][Pistache] Generated CMakeLists.txt sets compiler flag c++11 but should set c++14

Created by: cjuenger

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?
• [Optional] Bounty to sponsor the fix (example)

Description

The cpp-pistache-server generator generates a CMakeLists.txt file that sets c++11 compiler flag. As pistache do not supports c++11 anymore it should be changed to c++14.

openapi-generator version

4.2.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Some title
  description: Some description
  version: 0.0.0.0
  
servers:
  - url: http://localhost:8080 
  
paths:
  /doc:
    get:
      operationId: getDoc
      tags:
        - TAG
      security: []
      responses:
        '200':
          description: Documentation of the API.
          content:
            text/html:
              schema:
                type: string
        '500':
          description: Internal Server Error

Command line used for generation

openapi-generator generate -g cpp-pistache-server -i API_SPEC.yaml -o .\src -Dapis=TAG -Dmodels -DsupportingFiles

Steps to reproduce

Generate the cpp-pistache-server sources by executing the command line mentioned above.

Related issues/PRs

Suggest a fix

Change compiler flag from c++11 to c++14.