Description

Removes the unused @babel/runtime-corejs3 dependency from @docusaurus/babel.

Evidence this dependency is unused

1. The Babel preset explicitly disables corejs in transform-runtime:

   In packages/docusaurus-babel/src/preset.ts:

   [
     require.resolve('@babel/plugin-transform-runtime'),
     {
       corejs: false,  // <-- NOT using @babel/runtime-corejs3
       helpers: true,
       // ...
       absoluteRuntime: absoluteRuntimePath,  // points to @babel/runtime, not runtime-corejs3
     },
   ]

2. No source code imports from @babel/runtime-corejs3 - a code search only returns hits in CHANGELOG.md and package.json

3. The polyfill strategy uses @babel/preset-env with useBuiltIns: 'entry' and corejs: '3', which uses regular core-js (global polyfills), not @babel/runtime-corejs3 (sandboxed polyfills)

Why this matters

@babel/runtime-corejs3 depends on core-js-pure, which executes a postinstall script during npm install. Removing unused dependencies that run install scripts helps reduce supply chain attack surface.

Test plan

• Existing tests pass (the dependency is unused, so no behavior change expected)
• Build succeeds

Fixes #11585