fix(api-reference): remove Astro.canonicalURL

It was deprecated but it has been removed in v2.0.0 (see withastro/astro#5707). I
think it should not be displayed in the reference anymore since we are
now at v4.12.1

Author: ArmandPhilippot

src/content/docs/en/reference/api-reference.mdx

@@ -410,14 +410,6 @@ if (!isLoggedIn(cookie)) {
 ---
 ```
 
-### `Astro.canonicalURL`
-
-:::caution[Deprecated]
-Use [`Astro.url`](#astrourl) to construct your own canonical URL. 
-:::
-
-The [canonical URL][canonical] of the current page.
-
 ### `Astro.url`
 
 <p><Since v="1.0.0-rc" /></p>