Correct color naming for accessible combinations

Additions

Removed Black on Gray option for large type (discussed with @caheberer and we agreed that we don't want to encourage the use of this combination as it is difficult to read).

Corrected the color naming for the accessible combinations so that it maps correctly. I was using "green-20" and "green-60" but this didn't map. So, I changed back to "greenmid" and "greentint" (this is in <li class="type-color-combo__"). Since I can use the previous naming convention and still get the right visual result (and correct hex values) I'm moving forward with that approach).

Before:

After

@caheberer - I moved Gray on White and White on Gray to the accessible combinations since, according to the color contrast tool this is now accessible at all sizes.

[ ] Changes are limited to a single goal (no scope creep)
[ ] Code can be automatically merged (no conflicts)
[ ] Code follows the standards laid out in the front end playbook
[ ] Passes all existing automated tests
[ ] New functions include new tests
[ ] New functions are documented (with a description, list of inputs, and expected output)
[ ] Placeholder code is flagged
[ ] Visually tested in supported browsers and devices
[ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)