Fix off-by-one errors in purple deprecated color dark mode aliases

What does this MR do?

Fix off-by-one errors in the dark mode color aliases for deprecated purple color tokens, as identified in the review of !5906 (merged).

The dark mode inversion for deprecated purple color tokens was off by one step compared to the pattern used for other colors (blue, green, orange, red). The correct inversion pattern maps each scale step N to its mirror (approximately 950-N):

Token	Before	After
purple.50 dark	{color.purple.900}	{color.purple.950}
purple.100 dark	{color.purple.800}	{color.purple.900}
purple.200 dark	{color.purple.700}	{color.purple.800}
purple.300 dark	{color.purple.600}	{color.purple.700}
purple.400 dark	{color.purple.500}	{color.purple.600}

This MR is a follow-up to !5906 (merged) and addresses #3472 (closed)

Screenshots or screen recordings

The CI pipeline will update the image snapshot for the deprecated dark purple tokens to reflect the corrected values.

Integrations

n/a

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