Map border.tertiary to border.default

[simurai]

This maps the deprecated border.tertiary to border.default.

Reasoning

This came up in primer/css#1615. The history seems to be:

• In pre primitives border-gray-dark was the "strongest", most contrasting border color
• In primitives v1 it got renamed to color-border-tertiary which probably suggests it's the "weakest", but the color was still stronger than the default border.
• So because in primitives v2 the default border is the strongest, I think it would make sense to use this mapping:

.border-gray-dark -> .color-border-tertiary -> .color-border-default

Here a screenshot of color-border-tertiary from v1:

Keeping the current color-neutral-subtle would make the border barely visible:

We might also want to change all the border-tertiary -> neutral-muted migrations in https://github.com/github/github/pull/192360 to border-default. E.g. the border for this "Load more" button would be pretty hard to see:

/cc @ashygee

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Latest commit: ba564a3

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

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@primer/primitives	Patch

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