[php-slim4] Set required PHP version to 7.2 (#6530)

* Bump required PHP version to 7.2

* Update rest dependencies to meet php 7.2

Latest phpunit 9 requires PHP 7.3, so I've set phpunit 8 as a fallback.

* Fix TestCase inheritance

* Add phpunit cache file to gitignore

* Put license @phpdoc into separate mustache

* Bump readme PHP version to 7.2

* Bump @phpdoc PHP version to 7.2

* Update Zend Diactoros with suggested package

* Refresh samples

* Remove broken tests

These tests will be fixed in next PR which moves Mock feature to
external repo.

* Point root Travis CI environment to PHP 7.3

samples/server/petstore/php-slim4/lib/Model/AdditionalPropertiesObject.php

@@ -1,15 +1,20 @@
 <?php
 
 /**
- * AdditionalPropertiesObject
+ * OpenAPI Petstore
+ * PHP version 7.2
  *
- * PHP version 7.1
- *
- * @package OpenAPIServer\Model
+ * @package OpenAPIServer
  * @author  OpenAPI Generator team
  * @link    https://github.com/openapitools/openapi-generator
  */
 
+/**
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ * The version of the OpenAPI document: 1.0.0
+ * Generated by: https://github.com/openapitools/openapi-generator.git
+ */
+
 /**
  * NOTE: This class is auto generated by the openapi generator program.
  * https://github.com/openapitools/openapi-generator