Add new yellow utilities and variables

[broccolini]

Started this pr because I was testing something, @simurai or @shawnbot could you take this over and ship? Not urgent but fairly low effort.

Ref https://github.com/github/design-systems/issues/596

The bg-yellow-dark utility and variable to replace bg-pending, and text-yellow utility and variable to replace text-pending. These are using in a fair few places in dotcom so will need to update there before deprecating.

To do:

• update docs
• update stories (if we need to?)
• add deprecation warning so can deprecate in a major release

[simurai]

Docs update:

Utility	Screenshot
.text-yellow
.bg-yellow-dark
.bg-yellow-dark

[simurai]

Ok, I think this is ready for the next minor (12.3.0) release.

[shawnbot]

👌