[Bug] [Flink] Cannot convert field XXX from type BIGINT to STRING of Paimon table XXX

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Paimon version

0.9

### Compute Engine

Flink 1.20.0

### Minimal reproduce step

1. Synchronize mysql tables to paimon using cdc
2. Change the BIGINT type of the mysql table field to VARCHAR

### What doesn't meet your expectations?

Schema evolution failed

### Anything else?

Want to know the reason for this limitation ?

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Reproduce the CDC schema change with Paimon 0.9 and Flink 1.20. Start by tracing the schema-evolution handling for a BIGINT-to-VARCHAR change, then determine whether the failure is an intentional limitation or a defect. Done means the behavior is explained and, if appropriate, the supported conversion or required change is clearly identified.

Written by the indexing model from the issue text.