Fix alter column type JSON to nVarchar

[atiq-bs23]

Fixes #38364

SQL Server's native json type cannot be implicitly converted to nvarchar in ALTER COLUMN. When the old column type is json, the generator now uses rename → add → CONVERT-copy → drop instead.

• I've read the guidelines for contributing and seen the walkthrough
• I've posted a comment on an issue with a detailed description of how I am planning to contribute and got approval from a member of the team
• The code builds and tests pass locally (also verified by our automated build checks)
• Commit messages follow this format:
• Tests for the changes have been added (for bug fixes / features)
• Code follows the same patterns and style as existing code in this repo

[atiq-bs23]

@dotnet-policy-service agree