Upgrade to Astro 6.1.9 and migrate to content layer API

Moves content config from src/content/config.ts to src/content.config.ts
with glob loaders as required by Astro 6. Replaces entry.slug with entry.id
and entry.render() with render(entry) throughout. Removes legacy collections
option from astro.config.mjs.

src/pages/entries/[...slug].astro

@@ -1,18 +1,18 @@
 ---
-import { type CollectionEntry, getCollection } from 'astro:content';
+import { type CollectionEntry, getCollection, render } from 'astro:content';
 import Entry from '../../layouts/Entry.astro';
 
 export async function getStaticPaths() {
 	const entries = await getCollection('entries');
 	return entries.map((entry) => ({
-		params: { slug: entry.slug },
+		params: { slug: entry.id },
 		props: entry,
 	}));
 }
 type Props = CollectionEntry<'entries'>;
 
 const entry = Astro.props;
-const { Content, remarkPluginFrontmatter } = await entry.render();
+const { Content, remarkPluginFrontmatter } = await render(entry);
 entry.data.frontmatter = remarkPluginFrontmatter;
 ---