[mui-system] Fix darken() function

[arronhunt]

• I have followed (at least) the PR section of the contributing guide.

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Resolves #34136 by using the correct algorithm for darkening colors that will produce consistent results regardless of color format. Previously, a color darkened by a coefficient would produce different results if the color was an rgb color vs. hsl color.

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Another natural side effect is that default colors will be more vibrant.

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[mui-bot]

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@material-ui/core: parsed: +0.09% , gzip: +0.13%
@material-ui/lab: parsed: +0.18% , gzip: +0.25%
@material-ui/system: parsed: +0.66% , gzip: +0.80%
@mui/material-next: parsed: +0.12% , gzip: +0.15%

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[ZeeshanTamboli]

@siriwatknp Can you review this? To me it seems like a breaking change for primary and secondary dark colors.

[siriwatknp]

To me, this fix is a breaking change that should be considered in v7. What do you think @DiegoAndai ?

[DiegoAndai]

Thanks, @arronhunt, for the contribution.

I agree with @siriwatknp and @ZeeshanTamboli. This would be perceived as a breaking change. v7, which is expected to bring design improvements, is the best time for this fix.