fix(brand): Fix and improve Bootstrap variable handling

[gadenbuie]

1. Adding missing g flag in the regex that sanitizes brand.color.palette names for use in Sass and CSS variables.

2. Drop gray and gray-dark from Bootstrap color variables list. These appear in Bootstrap's $colors map but are derived from the gray color palette, and are not top-level color Sass variables.

3. Drop the brand.defaults.bootstrap.version check. On second thought, I imagine Quarto plans to support at most one version of Bootstrap at a time. You might want to throw a warning if someone asks for BS 5 after Quarto moves to BS 6 (in some distant future), but we don't need the version check now.

   Furthermore, in the defaults layer there's very little risk in setting Sass variables across Boostrap versions. Since they're all defined with ${var}: {value} !default;, either they'll be used by Bootstrap or they won't. So the chance of breakage is small.