Update dependency postcss-cli to v7

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
postcss-cli	dependencies	major	^5.0.1 -> ^7.0.0

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Release Notes

postcss/postcss-cli

v7.1.0

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• Respect map.annotation option when set to a string (#​307)

v7.0.0

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• BREAKING: Drop support for Node 6 & 8
• Update globby, which may be a breaking change for a small number of users using certain glob patterns, see https://github.com/mrmlnc/fast-glob#pattern-syntax for details (#​303)
• Update dependencies

v6.1.3

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• Fix external sorcemap filenames when the directory name contains .css (#​283, #​286)

v6.1.2

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• Upgrade dependency globby to fix bug in sub-dependency (#​265, #​270)

v6.1.1

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• Fix empty lines being printed in some cases (#​256, #​257)

v6.1.0

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• Add --include-dotfiles flag to allow globs to match dotfiles. (#​254)
• Only log warning messages. This has always been intended behavior, but it wasn't working as expected. (#​244)

v6.0.1

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• Better error handling for errors thrown by plugins (#​242, #​243)
• Update dependencies
• Clarify docs (#​233)

v6.0.0

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• Drop support for Node 4
• Upgrade to postcss v7 (release notes)
• Upgrade to postcss-load-config v2 (release notes)

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