Remove "v3" naming (admin styles) from the code base

[dacook]

What we should change and why (this is tech debt)

An admin_v3 naming has been introduced in the codebase, to distinguish the new admin styles from the legacy version, during the time period both versions existed in the code. The legacy version and feature toggle have been removed in the meantime, but we still have references to v3 which are not necessary anymore. We should remove these.

For example,
admin_v3 styles should be merged into admin (see all.scss)

Context

While looking at the codebase.

• See also Remove "v3" naming (product page) from the code base #13563

Impact and timeline

Improves clarity and code readability 🧹