Update dependency autoprefixer to v9.7.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
autoprefixer	dependencies	minor	9.6.5 -> 9.7.0

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Release Notes

postcss/autoprefixer

### [`v9.7.0`](https://togithub.com/postcss/autoprefixer/releases/9.7.0) [Compare Source](https://togithub.com/postcss/autoprefixer/compare/9.6.5...9.7.0) Autoprefixer 9.7 brings `AUTOPREFIXER_GRID` environment variable and huge performance improvments. #### Grid Environment Variable Autoprefixer can add [CSS Grid polyfills for IE](https://togithub.com/postcss/autoprefixer#does-autoprefixer-polyfill-grid-layout-for-ie). You need to manually enable it to prove that you understand the limits of polyfill. In previous versions you can enable Grid polyfill by `grid: 'autoplace'` option or by `/* autoprefixer grid: autoplace */` control comment. But in some cases, developers were not able to use both methods. For instance, during [work with Material UI’ CSS-in-JS in Create React App](https://togithub.com/postcss/autoprefixer/issues/1257). Now Autoprefixer supports environment variable to enable CSS Grid polyfill: ```sh AUTOPREFIXER_GRID=autoplace npm build ``` #### Performance Improvements [@​Knagis](https://togithub.com/Knagis) [found](https://togithub.com/postcss/autoprefixer/issues/1256) that Browserslist takes 70% of the Autoprefixer time.  We did a lot of improvements in Browserslist and now Autoprefixer should work much faster.  #### Other - Fix [`Cannot read property 'grid' of undefined`](https://togithub.com/postcss/autoprefixer/issues/1244) error

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[jaredlockhart]

bors r+

[bors[bot]]

:-1: Rejected by too few approved reviews

[mythmon]

bors merge

[bors[bot]]

Build succeeded

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