Revert "Preserve case for RowType's field name and JSON content when CAST"

[ajaygeorge]

This reverts commit d5a31b3.

Description

This reverts the commit d5a31b3.
which was causing issues with user queries.
See discussion in #21602 (comment)

Test Plan

Unit Tests

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[ajaygeorge]

@hantangwangd Let us add a legacy_json_cast flag in #21602 to preserve the original behavior since this is impacting user queries.

[NikhilCollooru]

Having a flag to control a changed behavior is better. Approving this revert. @hantangwangd please send a new PR that provides a flag to control this . Thanks !

[jainxrohit]

LGTM

[hantangwangd]

OK, I will do this. Thanks for your suggestion. @ajaygeorge @NikhilCollooru