Update dependency postcss-cssnext to v3.1.1 - autoclosed

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This PR contains the following updates:

Package	Type	Update	Change
postcss-cssnext (source)	dependencies	patch	3.1.0 -> 3.1.1

By merging this PR, the issue #1422 will be automatically resolved and closed:

Severity	CVSS Score	CVE
High	7.5	WS-2021-0152
Medium	5.3	CVE-2021-29060

---

• If you want to rebase/retry this PR, click this checkbox.

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⚠ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

• any of the package files in this branch needs updating, or
• the branch becomes conflicted, or
• you click the rebase/retry checkbox if found above, or
• you rename this PR's title to start with "rebase!" to trigger it manually

The artifact failure details are included below:

File name: yarn.lock

error @1.1.0: The engine "node" is incompatible with this module. Expected version ">= 12.0.0 < 15". Got "16.15.1"
error Found incompatible module.

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