V2 last minute tweaks

[simurai]

This makes the following changes to the deprecated/migration guide:

Replace .color-border-tertiary

- .color-border-tertiary -> .color-border-muted
+ .color-border-tertiary -> .color-border-default

Depends a bit on primer/primitives#242.

Replace .color-text-tertiary

- .color-text-tertiary -> .color-fg-subtle
+ .color-text-tertiary -> .color-fg-muted

Can't remember if that was intentional or just an oversight. But it seems that Primitives uses "text.tertiary": "fg.muted",. So maybe better to keep it the same for the utilities.

[changeset-bot]

🦋 Changeset detected

Latest commit: bab09f3

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	Major

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[simurai]

The history seems to be:

• In v15 .border-gray-dark was the "strongest", most contrasting border color
• In v16 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 v18 the default border is the strongest, I think it would make sense to use this mapping:

.border-gray-dark (v15) -> .color-border-tertiary (v16) -> .color-border-default (v18)

Maybe we should change that in primitives too, even though the variables already changed on dotcom https://github.com/github/github/pull/192360. Here a screenshot from v16: