[BUG] [multiplatform] unintentionally convert a datetime without a time zone to a datetime with UTC time zone.

Description

The generator unintentionally convert a datetime without a time zone to a datetime with UTC time zone.

write in yaml

          dateTime2:
            type: string
            example: "2021-09-01 00:00:00"

but web ui shows:
"dateTime2": "2021-09-01T00:00:00.000Z"

At least, there are the problem with "python-flask" and "nodejs-express-server" .
but there aren't the problem with "html".

openapi-generator version

5.2.1.

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: test
  version: 1.0.0
paths:
  /test/{dateTime}/:
    get:
      parameters:
        - name: "dateTime"
          in: "path"
          required: true
          schema:
            type: string
            example: "2021-09-01 00:00:00"
      responses:
        "200":
          description: Success
          content:
            application/json:
              schema:
                type: object
                properties:
                  dateTime2:
                    type: string
                    example: "2021-09-01 00:00:00"

Generation Details

java -jar openapi-generator-cli-5.2.1.jar generate -i test.yaml -g python-flask -o test-openapi-flask

java -jar openapi-generator-cli-5.2.1.jar generate -i test.yaml -g nodejs-express-server -o test-openapi-nodejs-express-server

java -jar openapi-generator-cli-5.2.1.jar generate -i test.yaml -g html -o test-openapi-html

Steps to reproduce

java -jar openapi-generator-cli-5.2.1.jar generate -i test.yaml -g python-flask -o test-openapi-flask
cd test-openapi-flask
python -m openapi_server

access
http://localhost:8080/ui/#/default/test_date_time_get

Related issues/PRs

https://app.slack.com/client/TLQFRCNJZ/CLSB0U0R5

Suggest a fix

Start with the provided OpenAPI 3.0.3 YAML and the generation commands for python-flask and nodejs-express-server, then compare their output with the html generator. Trace how the example value is handled in the generated servers; done means a timezone-less datetime remains timezone-less in the affected outputs while the reproduction still works.