Update dependency html-minifier to v3.5.21

This MR contains the following updates:

Package	Change	Age	Confidence
html-minifier (source)	3.5.15 -> 3.5.21

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Release Notes

kangax/html-minifier (html-minifier)

v3.5.21

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v3.5.20

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v3.5.19

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v3.5.18

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v3.5.17

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v3.5.16

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