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/content/config.ts

@@ -1,32 +0,0 @@
-import { defineCollection, z } from 'astro:content';
-
-const blog = defineCollection({
-	// Type-check frontmatter using a schema
-	schema: z.object({
-		title: z.string(),
-		description: z.string(),
-		// Transform string to Date object
-		pubDate: z.coerce.date(),
-		updatedDate: z.coerce.date().optional(),
-		heroImage: z.string().optional(),
-	}),
-});
-
-const entries = defineCollection({
-	// Type-check frontmatter using a schema
-	schema: z.object({
-		title: z.string(),
-		// Transform string to Date object
-		pubDate: z.coerce.date().optional(),
-		updatedDate: z.coerce.date().optional(),
-	}),
-});
-
-const index = defineCollection({
-	// Type-check frontmatter using a schema
-	schema: z.object({
-		title: z.string(),
-	}),
-});
-
-export const collections = { blog, entries, index };