[php-slim4] Switch to Packaged Body Parsing Middleware

Created by: ybelenko

Is your feature request related to a problem? Please describe.

Custom JSON parsing middleware isn't necessary anymore. Latest version of PHP Slim4 already contains Body Parsing Middleware

Describe the solution you'd like

1. Remove custom JSON parsing middleware file from generation.
2. Enable packaged Body Parsing Middleware

I will take care of it after #9208 merge to avoid conflicts and double review.