chore(deps): update dependency cssnano to v5.1.15

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
cssnano	5.1.13 → 5.1.15

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Release Notes

cssnano/cssnano (cssnano)

v5.1.15: v5.1.15

Compare Source

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

v5.1.14: v5.1.14

Compare Source

Bug Fixes

• fix: update autoprefixer and browserslist
• fix(postcss-reduce-initial): improve initial properties data

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