Use `control` borders for subnav + disabled button fix

[langermank]

• Use control borders to help with updating contrast in the future
• Fix disabled button borders

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: d9294931f03fd1550a97f6fac547c017463e51e2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ----------- | ----- | | @primer/css | Patch |

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