[SPARK-38211][SQL][DOCS] Add SQL migration guide on restoring loose upcast from string to other types
[SPARK-38211][SQL][DOCS] Add SQL migration guide on restoring loose upcast from string to other types

### What changes were proposed in this pull request?
Add doc on restoring loose upcast from string to other types (behavior before 2.4.1) to SQL migration guide.

### Why are the changes needed?
After [SPARK-24586](https://issues.apache.org/jira/browse/SPARK-24586), loose upcasting from string to other types are not allowed by default. User can still set `spark.sql.legacy.looseUpcast=true` to restore old behavior but it's not documented.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Only doc change.

Closes #35519 from manuzhang/spark-38211.

Authored-by: tianlzhang <870a6c6b0b4c7e903b13e70a9449dba4b745a4e6@ebay.com>
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

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