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.