Fix carousel caption heading colors

[julien-deramond]

Description

This PR suggests to set --bs-heading-color to black in dark mode or when .carousel-dark is used.
TBH I'm not a big fan of this modification so far but headers have a color set by --bs-heading-color which has a bigger priority compared to .carousel-caption color. So I haven't found a more elegant way of doing it for now. Thoughts to improve this PR?

Motivation & Context

Color consistency between header and text colors within .carousel-caption.

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• (NA) I have added tests to cover my changes
• All new and existing tests passed

Live previews

Please check the following URLs in light and dark modes:

• https://deploy-preview-37797--twbs-bootstrap.netlify.app/docs/5.3/components/carousel/#captions
• https://deploy-preview-37797--twbs-bootstrap.netlify.app/docs/5.3/components/carousel/#dark-variant

Related issues

Closes #37795

[mdo]

I think I just have to remove the heading color from dark mode. Causing too many issues it seems.

[julien-deramond]

Yep seems to be an issue in #37799 as well.
Let's try to evaluate if it's doable without this heading color. If we go for it, this PR can be closed.