remove two deprecated serverless plugins and update serverless to the version 3

[siruku6]

Target plugins

• serverless-reqvalidator-plugin
• serverless-aws-documentation

Currently

This app is using serverless@2.72.3.

Reference

• https://forum.serverless.com/t/how-do-you-add-a-request-validator-to-api-gateway/12923
• https://www.serverless.com/framework/docs/providers/aws/events/apigateway#request-parameters