Deprecate `getLastResponse...()` and `request...()` in Clients

[Art4]

Since v2.5.0 (#370) all Clients implements the method Redmine\Http\HttpClient::request() and with v2.6.0 (#378) all API classes will have the new method Redmine\Api\...::getLastResponse() to get the last response made by the API class. The users should now use this new getLastResponse() method. Therefore the following methods should be deprecated and removed.

• Redmine\Client\Client::requestGet()
• Redmine\Client\Client::requestPost()
• Redmine\Client\Client::requestPut()
• Redmine\Client\Client::requestDelete()
• Redmine\Client\Client::getLastResponseStatusCode()
• Redmine\Client\Client::getLastResponseContentType()
• Redmine\Client\Client::getLastResponseBody()

Follows #341.