[BUG][Openapi-yaml] Single quotes are deleted in examples

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

I use the generator openapi-yaml for merge serveral yaml files into one. The bug is that the generator remove de simple quotes in examples.

openapi-generator version

4.1.3, 4.2.0

OpenAPI declaration file content or url

Simplified input file:

openapi: 3.0.1
info:
  title: ping test
  version: '1.0'
servers:
  - url: 'http://localhost:8000/'
paths:
  /search:
    get:
      operationId: search
      parameters:
        - name: date
          in: query
          description: The date of the document.
          example: '2019-08-06T13:40:43.449Z'
          required: true
          schema:
            type: string
            format: date-time
      responses:
        '201':
          description: OK

Command line used for generation

java -jar openapi-generator-cli.jar generate -g openapi-yaml -i openapi.yml

Steps to reproduce

Run the above command
Expected output:

...
     parameters:
        - name: date
          in: query
          description: The date of the document.
          example: '2019-08-06T13:40:43.449Z'
...

Actual output:

...
     parameters:
        - name: date
          in: query
          description: The date of the document.
          example: 2019-08-06T13:40:43.449Z
...

Reproduce the issue with the openapi-yaml generator using the provided OpenAPI declaration and the Java command shown. Compare the generated YAML with the expected output, focusing on the quoted date-time example. Done means generation preserves the single quotes around that example.