Update dependency gulp-imagemin to v9

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
gulp-imagemin	~6.2.0 → ~9.2.0

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Release Notes

sindresorhus/gulp-imagemin (gulp-imagemin)

v9.2.0

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• Allow optimizing WebP images (#​393) 4aeb145

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v9.1.0

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• Update dependencies 127615e

v9.0.0

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Breaking

• Require Node.js 18 70e07e8

v8.0.0

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• Require Node.js 12.20 1b4baf6
• This package is now pure ESM. Please read this.
• The bundled plugins are now named exports instead of being attached to the default export.

v7.1.0

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• Update imagemin-gifsicle optional dependency 67cceb3

v7.0.0

Compare Source

Breaking

• Require Node.js 10 aacca91
• Replace jpegtran with mozjpeg in the default plugins (#​336) 279a91b

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