fix(highlight-ssr): migrate to rehype-highlight@6

Author: pd4d10

packages/plugin-highlight-ssr/src/index.ts

@@ -2,12 +2,11 @@ import type { BytemdPlugin } from 'bytemd'
 import rehypeHighlight, { Options } from 'rehype-highlight'
 
 export default function highlightSsr({
-  subset = false,
   ignoreMissing = true,
   ...rest
 }: Options = {}): BytemdPlugin {
   return {
     rehype: (processor) =>
-      processor.use(rehypeHighlight, { subset, ignoreMissing, ...rest }),
+      processor.use(rehypeHighlight, { ignoreMissing, ...rest }),
   }
 }