Revert "build(core): enable advanced ESM (#6513)"

[chenjiahan]

Summary

This reverts PR #6513 because the advancedEsm introduced some unexpected exports:

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR removes the experimental advancedEsm configuration from three ESM format build configurations in the core package's rslib config file. The change simplifies the build configuration by removing this experimental feature flag.

• Removes experiments.advancedEsm: true from esm_index, esm_loaders, and esm_client configurations

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