Update postcss-preset-env to avoid build warnings

[Turbo87]

I've been seeing the following warning in my builds for a while:

┌─────────────────────────────────────────────────────────────────────────────────┐
│                                                                                 │
│   This version of postcss-preset-env is not optimised to work with PostCSS 8.   │
│                Please update to version 7 of PostCSS Preset Env.                │
│                                                                                 │
│                    If you find issues, you can report it at:                    │
│          https://github.com/csstools/postcss-plugins/issues/new/choose          │
│                                                                                 │
└─────────────────────────────────────────────────────────────────────────────────┘

It looks like #320 updated the version of broccoli-postcss from v5 to v6, which then updated PostCSS under the hood from v7 to v8. The postcss-preset-env dependency of this project however was not updated and was still pointing to a version that was causing the above warning.

This PR updates the dependency to the latest version, which is compatible with PostCSS v8. The changelog for the diff is available at https://github.com/csstools/postcss-plugins/blob/main/plugin-packs/postcss-preset-env/CHANGELOG.md. I did not find any breaking changes that would be relevant to this project though, so no code changes on our side were necessary.

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[Turbo87]

#362 should hopefully resolve the compilation issue...

[Turbo87]

@mansona rebasing fixed the compilation issue :)

[nlepage]

Hi @mansona
any chance this would be merged and released soon?