Update dependency postcss to v8.5.15

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
postcss (source)	8.5.6 → 8.5.15

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Release Notes

postcss/postcss (postcss)

v8.5.15

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• Fixed declaration parsing performance (by @​homanp).

v8.5.14

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• Fixed custom syntax regression (by @​43081j).

v8.5.13

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• Fixed postcss-scss commend regression.

v8.5.12

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• Fixed reading any file via user-generated CSS.
• Added opts.unsafeMap to disable checks.

v8.5.11

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• Fixed nested brackets parsing performance (by @​offset).

v8.5.10

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• Fixed XSS via unescaped </style> in non-bundler cases (by @​TharVid).

v8.5.9

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• Speed up source map encoding paring in case of the error.

v8.5.8

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• Fixed Processor#version.

v8.5.7

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• Improved source map annotation cleaning performance (by CodeAnt AI).

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