Update dependency autoprefixer to v9.6.4

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
autoprefixer	dependencies	patch	9.6.1 -> 9.6.4

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Release Notes

postcss/autoprefixer

### [`v9.6.4`](https://togithub.com/postcss/autoprefixer/blob/master/CHANGELOG.md#​964) [Compare Source](https://togithub.com/postcss/autoprefixer/compare/9.6.3...9.6.4) - Now the real fix for `'startsWith' of undefined` error. ### [`v9.6.3`](https://togithub.com/postcss/autoprefixer/blob/master/CHANGELOG.md#​963) [Compare Source](https://togithub.com/postcss/autoprefixer/compare/9.6.2...9.6.3) - Fix `Cannot read property 'startsWith' of undefined` error. ### [`v9.6.2`](https://togithub.com/postcss/autoprefixer/blob/master/CHANGELOG.md#​962) [Compare Source](https://togithub.com/postcss/autoprefixer/compare/9.6.1...9.6.2) - Fix false `Replace fill to stretch` warning.

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