Update dependency postcss-loader to v6

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss-loader	^5.2.0 -> ^6.0.0

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Release Notes

webpack-contrib/postcss-loader

v6.2.1

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v6.2.0

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Features

• add link field in schema (#​540) (1ae8212)

6.1.1 (2021-07-01)

Bug Fixes

• do not swallow exception from postcss (2eec42b)

v6.1.1

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v6.1.0

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Features

• allow String value for the "implementation" option (0d342b1)

v6.0.0

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⚠ BREAKING CHANGES

• minimum supported Node.js version is 12.13.0 (#​526)

Bug Fixes

• check postcss as project dependency (570db67)

v5.3.0

Compare Source

Features

• add support for dir-dependency message type (#​524) (91dea60)

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