Twenty Twenty-Four: Change font family slug to lowercase.

Ensures referencing the correct CSS custom property.

Props RavanH, poena, onemaggie, huzaifaalmesbah, mukesh27.
Fixes #60325.

git-svn-id: https://develop.svn.wordpress.org/trunk@57351 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-content/themes/twentytwentyfour/theme.json

@@ -247,7 +247,7 @@
 				{
 					"fontFamily": "Iowan Old Style, Apple Garamond, Baskerville, Times New Roman, Droid Serif, Times, Source Serif Pro, serif, Apple Color Emoji, Segoe UI Emoji, Segoe UI Symbol",
 					"name": "System Serif",
-					"slug": "system-Serif"
+					"slug": "system-serif"
 				}
 			],
 			"fontSizes": [