Update dependency autoprefixer to v10.4.21

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
autoprefixer	10.4.14 -> 10.4.21

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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).

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