Also removed last occurence of 1.6 prerelease callout

(cherry picked from commit 5b44892)

Author: cderv

docs/presentations/revealjs/index.qmd

@@ -365,8 +365,6 @@ Note that if the background color of your media differs from your presentation's
 
 ### Gradient Background
 
-{{< include /docs/prerelease/1.6/_pre-release-feature.qmd >}}
-
 All CSS gradient formats are supported, including `linear-gradient`, `radial-gradient` and `conic-gradient`.
 
 ```{.markdown code-preview="examples/background-gradient.qmd"}