Update post max size

[jbrown-xentity]

Related to https://github.com/GSA/datagov-deploy/issues/3686

For those that utilize the dashboard validator "upload" feature.

Pulled from:

• https://docs.cloudfoundry.org/buildpacks/php/gsg-php-config.html
• https://www.php.net/manual/en/ini.core.php#ini.post-max-size
• https://stackoverflow.com/questions/6279897/post-content-length-exceeds-the-limit#:~:text=8388608%20bytes%20is%208M%2C%20the,need%20to%20be%20changed%20here.