fix(button): add explicit highcontrast variables for disabled color

[mlogsdon18]

Description

In SWC, the highcontrast text color for disabled buttons was incorrect. We were not explicitly assigning that color variable, so this PR adds --highcontrast variables for disabled color and border color.

SWC Github Issue

How and where has this been tested?

• How this was tested: Tested using yarn link to verify that the change does indeed make the desired update in SWC

Screenshots

Current

This PR

To-do list

• If my change impacts other components, I have tested to make sure they don't break.
• If my change impacts documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have tested these changes in Windows High Contrast mode.
• I have updated any relevant storybook stories and templates.
• If my change(s) include visual change(s), a designer has reviewed and approved those changes.

• This pull request is ready to merge.

[github-actions]

🚀 Deployed on https://pr-1985--spectrum-css.netlify.app

[pfulton]

This mostly looks good, but when I view the "button - accent" page in the docs site, it looks like the font color on the default variants were negatively impacted by the changes.

[mlogsdon18]

@pfulton ope I don't know how I missed that! Should be good to go now, I needed that forced-color-adjust: none on the label