Add new border-color and background-color utilities

Pulling out this commit from #30778 which is outdated and kind of a mess.

• bg-gray is new and maps to gray-400
• border-gray-500 and border-gray-700 are new and are meant to complement bg-gray and bg-secondary as one level off from the borders

Still TBD if we want it, but figured I'd separate this out for now to see if we want/need it.

Preview:

• https://deploy-preview-31694--twbs-bootstrap.netlify.app/docs/5.0/utilities/borders/#border-color
• https://deploy-preview-31694--twbs-bootstrap.netlify.app/docs/5.0/utilities/colors/#background-color