Bump postcss from 8.4.18 to 8.4.20

[dependabot[bot]]

Bumps postcss from 8.4.18 to 8.4.20.

Release notes

Sourced from postcss's releases.

8.4.20

• Fixed source map generation for childless at-rules like @layer.

8.4.19

• Fixed whitespace preserving after AST transformations (by @​romainmenke).

Changelog

Sourced from postcss's changelog.

8.4.20

• Fixed source map generation for childless at-rules like @layer.

8.4.19

• Fixed whitespace preserving after AST transformations (by Romain Menke).

Commits

• 1d4c509 Release 8.4.20 version
• 905082a Add Node.js 10 to CI
• 20122e5 Fix childless at-rule map generation
• ff7e8ce Merge pull request #1801 from romainmenke/invalid-sourcemap-with-at-layer--pr...
• d585976 Fix CI
• 744b15f Update dependencies
• db935f1 invalid sourcemap with at layer
• 3cac84a Fix type tests
• 5113edb Update dependencies
• c26baf3 Release 8.4.19 version
• Additional commits viewable in compare view

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[dependabot[bot]]

Looks like postcss is up-to-date now, so this is no longer needed.