chore(beep boop 🤖): Bump `MCORE_TAG=b6866ae...` (2024-11-20)
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🚀 PR to bump `NVIDIA/Megatron-LM` in `Dockerfile.ci` to `MCORE_TAG=b6866aed34d28fc37e352f3bae53fa48ad150952`.  

📝 Please remember the following to-do's before merge: 
- [ ] Verify the presubmit CI  

🙏 Please merge this PR only if the CI workflow completed successfully.