Update postcss-loader version

[Defite]

Update postcss-loader in order to use postcss-preset-env with gatsby-plugin-postcss. This will update postcss to v7 as well.

Example of postcss-preset-env config.

require(`postcss-preset-env`)(
            { 
              stage: 2,
              features: {
                'nesting-rules': true
              }
            }
)

[m-allanson]

Thanks @Defite 👍

[gatsbot]

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