Bug - Bar charts - update border to alt token in high contrast

[mcoker]

patternfly/patternfly-react#12419 (review)

from @lboehling

One small nit -- can we update the stroke around the bar charts to use border--color--alt instead of high-contrast border color? (regular & stacked -- i think they reference the same var, i just can't find it in the diff)

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Jira Issue: PF-4163

[coderabbitai]

🔗 Related PRs

#8241 - fix(progress): apply unified theme and icons [merged]
#8248 - fix(jump-links): adjust accent line for unified theme [merged]
#8301 - feat(card): update clickable/selectable plain card border styles [merged]
#8379 - feat(charts): add high contrast [merged]
#8396 - fix: update bar chart border [closed]

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