Update dependency cssnano to v5.1.15

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
cssnano	5.1.14 -> 5.1.15

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Release Notes

cssnano/cssnano

### [`v5.1.15`](https://togithub.com/cssnano/cssnano/releases/tag/cssnano%405.1.15): v5.1.15 [Compare Source](https://togithub.com/cssnano/cssnano/compare/cssnano@5.1.14...cssnano@5.1.15) #### Bug Fixes - fix(postcsss-reduce-initial): fix mask-repeat conversion - fix(postcss-colormin): don't minify colors in src declarations - fix(postcss-merge-rules): do not merge conflicting flex and border properties

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Configuration

📅 Schedule: Branch creation - "every weekday" (UTC), Automerge - At any time (no schedule defined).

🚦 Automerge: Enabled.

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🔕 Ignore: Close this PR and you won't be reminded about this update again.

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