fix(richtext-lexical): bump acorn to resolve type mismatch with transitive dep (#15791)

# Overview

Fixes the `richtext-lexical` build failure caused by a type
incompatibility between two resolved versions of `acorn`.

**Bump acorn from 8.12.1 to 8.16.0:** `acorn@8.16.0` was published on
Feb 19, 2026 and added `ecmaVersion: 17 | 2026` to its type union.
`micromark-extension-mdx-jsx` depends on `acorn` via a `^8.x` range and
resolved to 8.16.0, while `richtext-lexical` pinned 8.12.1. TypeScript
treats the `Options` types from each version as distinct, so passing the
8.12.1 `acorn` module into `mdxJsx()` (which expects 8.16.0 types) fails
at build time. Bumping the direct dep to 8.16.0 deduplicates both to a
single version.

Author: denolfe

packages/richtext-lexical/package.json

@@ -381,7 +381,7 @@
     "@payloadcms/translations": "workspace:*",
     "@payloadcms/ui": "workspace:*",
     "@types/uuid": "10.0.0",
-    "acorn": "8.12.1",
+    "acorn": "8.16.0",
     "bson-objectid": "2.0.4",
     "csstype": "3.1.3",
     "dequal": "2.0.3",