Consolidate rgb and rgba into single function (rgb)

[SamuelMiller]

Prerequisites

• [X] I have searched for duplicate or closed feature requests
• [X] I have read the contributing guidelines

Proposal

According to this article (https://dev.to/alvaromontoro/the-ultimate-guide-to-css-colors-2020-edition-1bh1), rather than two separate color functions rgb and rgba (the same for hsl and hsla), the newer simplified format drops the ending alpha "a" suffix and instead uses forward slash "/", merging the two separate functions into one. For example,

/* old notation */
color: rgb(255, 255, 255);
color: rgba(255, 255, 255, 1);

/* new notation */
color: rgb(255 255 255);
color: rgb(255 255 255 / 1);

I suggest Bootstrap consider implementing the newer notation.

Motivation and context

Simplify code. Keep up with the times, man.

[mdo]

This could be something we tackle in v6. Syntax changes like this could easily break things for folks, and they'd change our functionality for sure on existing color variables.

[beetrandahiya]

Why can't they be merged using the same "," notation, by using the arguments object?

[mdo]

The point I was making is I'd want to update our CSS variables and general rgba() usage to both use comma-less syntax. Switching syntax for variable values mid-version is a no-no.