Update dependency postcss to v8.4.20

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss (source)	`8.4.6` -> `8.4.20`

Release Notes

postcss/postcss

### [`v8.4.20`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8420) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.19...8.4.20) - Fixed source map generation for childless at-rules like `@layer`. ### [`v8.4.19`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8419) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.18...8.4.19) - Fixed whitespace preserving after AST transformations (by Romain Menke). ### [`v8.4.18`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8418) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.17...8.4.18) - Fixed an error on `absolute: true` with empty `sourceContent` (by Rene Haas). ### [`v8.4.17`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8417) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.16...8.4.17) - Fixed `Node.before()` unexpected behavior (by Romain Menke). - Added TOC to docs (by Mikhail Dedov). ### [`v8.4.16`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8416) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.15...8.4.16) - Fixed `Root` AST migration. ### [`v8.4.15`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8415) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.14...8.4.15) - Fixed AST normalization after using custom parser with old PostCSS AST. ### [`v8.4.14`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8414) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.13...8.4.14) - Print “old plugin API” warning only if plugin was used (by [@zardoy](https://togithub.com/zardoy)). ### [`v8.4.13`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8413) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.12...8.4.13) - Fixed `append()` error after using `.parent` (by Jordan Pittman). ### [`v8.4.12`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8412) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.11...8.4.12) - Fixed `package.funding` to have same value between all PostCSS packages. ### [`v8.4.11`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8411) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.10...8.4.11) - Fixed `Declaration#raws.value` type. ### [`v8.4.10`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#8410) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.9...8.4.10) - Fixed `package.funding` URL format. ### [`v8.4.9`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#849) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.8...8.4.9) - Fixed `package.funding` (by Álvaro Mondéjar). ### [`v8.4.8`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#848) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.7...8.4.8) - Fixed end position in empty Custom Properties. ### [`v8.4.7`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#847) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.6...8.4.7) - Fixed `Node#warn()` type (by Masafumi Koba). - Fixed comment removal in values after `,`.