Fix $font-line-height-x-large error

[ajlende]

What?

Hardcode $font-line-height-x-large from #65418 which didn't get backported.

Why?

Wasn't sure if we wanted all the new variables in wp/6.7, so this is a fix without including all of them.

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

[jeryj]

My CSS builds now 👍🏻

[github-actions]

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[ajlende]

The Compressed Size / Check (pull_request) check will continue to fail because it's trying to build the previous commit which is broken, so we need someone with commit access to merge this one.

[ciampo]

we need someone with commit access to merge this one.

Done!