`@tailwindcss/upgrade` doesn't upgrade `extends.content` correctly

[simon-abbott]

What version of Tailwind CSS are you using?

v3.4.18

What build tool (or framework if it abstracts the build tool) are you using?

Not relevant

What version of Node.js are you using?

For example: v24.11.1

What browser are you using?

Not relevant

What operating system are you using?

macOS

Reproduction URL

https://play.tailwindcss.com/SZJTT4w0iF

Describe your issue

I am trying to upgrade from v3 to v4 using the auto-upgrade tool. I have a custom content value in my config. When I run the upgrade tool (npx @tailwindcss/upgrade) it migrates that setting over to be --content-slash, but then my linter errors because after:content-slash isn't a valid class.