Update dependency postcss to v8.5.15

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
postcss (source)	8.5.14 → 8.5.15

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Release Notes

postcss/postcss (postcss)

v8.5.15

Compare Source

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

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Configuration

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Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/javascript/BuiltInFormControls/AddFileModal.tsx	🔴 0%	🔴 33.33%	🟢 33.33%
app/javascript/BuiltInFormControls/LiquidInput.tsx	🟠 57.5%	🟠 60%	🟢 2.5%
Overall Coverage	🟢 53.06%	🟢 53.09%	🟢 0.03%

Minimum allowed coverage is 0%, this run produced 53.09%