chore(astro): upgrade to Astro 6 and move content config

Add @astrojs/rss and update blog, changelog, and RSS pages
to use the new content collections configuration path

src/pages/blog/[slug].astro

@@ -1,17 +1,17 @@
 ---
-import { getCollection } from 'astro:content';
+import { getCollection, render } from 'astro:content';
 import BlogPost from '../../layouts/BlogPost.astro';
 
 export async function getStaticPaths() {
   const posts = await getCollection('blog');
   return posts.map((post) => ({
-    params: { slug: post.slug },
+    params: { slug: post.id },
     props: post,
   }));
 }
 
 const post = Astro.props;
-const { Content } = await post.render();
+const { Content } = await render(post);
 ---
 
 <BlogPost post={post}>