Turbopack: Update to swc_core v50.2.3 (#87841)
Update to swc_core v50.2.3 to address a parse issue with mdx files that should be fixed by swc-project/swc#11372:
- Fixes #87713
- Internal discussion: https://vercel.slack.com/archives/C04KC8A53T7/p1766716756273609?thread_ts=1766712717.419489&cid=C04KC8A53T7
I'm intentionally not updating to the latest SWC here to reduce risk, as I'd like to cherry-pick this into the Next 16.1 branch.
Changelog: https://github.com/swc-project/swc/blob/f9b4da2bd85d160b3ee4b3296ed520388675b90e/CHANGELOG-CORE.md#swc_corev5023---2025-12-15
