[warning in terminal] Update PostCSS Preset Env to version 7

[christinach]

While deploying with capistrano it took a long time in yarn install.
One of the messages in the console:
This version of postcss-preset-env is not optimised to work with PostCSS 8. │
01 │ Please update to version 7 of PostCSS Preset Env.

[christinach]

I checked why this package is used: yarn why postcss-preset-env

yarn why postcss-preset-env      
yarn why v1.22.5
[1/4] 🤔  Why do we have the module "postcss-preset-env"...?
[2/4] 🚚  Initialising dependency graph...
[3/4] 🔍  Finding dependency...
[4/4] 🚡  Calculating file sizes...
=> Found "[email protected]"
info Reasons this module exists
   - "@rails#webpacker" depends on it
   - Hoisted from "@rails#webpacker#postcss-preset-env"
info Disk size without dependencies: "180KB"
info Disk size with unique dependencies: "10.08MB"
info Disk size with transitive dependencies: "12.76MB"
info Number of shared dependencies: 55
✨  Done in 0.67s.

We don't use webpacker anymore. I removed it from package.json and yarn install.