Update CSS preprocessors

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
autoprefixer	10.4.7 -> 10.4.21
cssnano	5.1.12 -> 5.1.15

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Release Notes

postcss/autoprefixer (autoprefixer)

v10.4.21

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• Fixed old -moz- prefix for :placeholder-shown (by @​Marukome0743).

v10.4.20

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• Fixed fit-content prefix for Firefox.

v10.4.19

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• Removed end value has mixed support, consider using flex-end warning
  since end/start now have good support.

v10.4.18

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• Fixed removing -webkit-box-orient on -webkit-line-clamp (@​Goodwine).

v10.4.17

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• Fixed user-select: contain prefixes.

v10.4.16

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• Improved performance (by Romain Menke).
• Fixed docs (by Christian Oliff).

v10.4.15

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• Fixed ::backdrop prefixes (by 一丝).
• Fixed docs (by Christian Oliff).

v10.4.14

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• Improved startup time and reduced JS bundle size (by Kārlis Gaņģis).

v10.4.13

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• Fixed missed prefixes on vendor prefixes in name of CSS Custom Property.

v10.4.12

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• Fixed support of unit-less zero angle in backgrounds (by 一丝).

v10.4.11

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• Fixed text-decoration prefixes by moving to MDN data (by Romain Menke).

v10.4.10

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• Fixed unicode-bidi prefixes by moving to MDN data.

v10.4.9

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• Fixed css-unicode-bidi issue from latest Can I Use.

v10.4.8

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• Do not print color-adjust warning if print-color-adjust also is in rule.

cssnano/cssnano (cssnano)

v5.1.15: v5.1.15

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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

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Bug Fixes

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

v5.1.13: v5.1.13

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Bug Fixes

• fix TypeScript type of cssnano() return value (cssnano/cssnano@b92dbe3)

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