fix(theme): forward className prop in theme-classic's Heading (#8350)

Fixes #8327

Author: JoshuaKGoldberg

packages/docusaurus-theme-classic/src/theme/Heading/index.tsx

@@ -30,6 +30,7 @@ export default function Heading({as: As, id, ...props}: Props): JSX.Element {
         hideOnScroll
           ? styles.anchorWithHideOnScrollNavbar
           : styles.anchorWithStickyNavbar,
+        props.className,
       )}
       id={id}>
       {props.children}