break(core): remove the @docusaurus/core/lib/babel/preset re-export (AI-assisted) (#12707)

The Babel preset moved to @docusaurus/babel in v3.6. Old sites with a
babel.config.js generated before v3.6 must use '@docusaurus/babel/preset'
or delete the file.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: slorber

knip.config.ts

@@ -86,7 +86,6 @@ const config: KnipConfig = {
         'src/client/exports/*.{ts,tsx}',
         'src/client/theme-fallback/**/*.tsx',
         'src/ssg/ssgWorkerThread.ts',
-        'src/babel/preset.ts',
         // Ambient declarations with triple-slash directives
         'src/**/*.d.ts',
       ],