Deprecate ExprSchemable::metadata

### Is your feature request related to a problem or challenge?

- Similar to https://github.com/apache/datafusion/issues/18844

ExprSchemable::metadata simply calls through to `to_field` and thus makes the API more complicated for no real gain

https://github.com/apache/datafusion/blob/4f95e6d70249d0d839939facc67f95cd7f54526e/datafusion/expr/src/expr_schema.rs#L421-L424

### Describe the solution you'd like

We should deprecate `metadata` to encourage people to use `to_field` directly

The deprecation guidelines are here: https://datafusion.apache.org/contributor-guide/api-health.html#deprecation-guidelines

### Describe alternatives you've considered

1. Add deprecation notice to `ExprSchemable::metadata`
2. Add a note / example in the docs for `ExprSchemable::to_field` that notes how to get Metadata
3. Update all existing code locations that call `ExprSchemable::metadata` to use `ExprSchemable::to_field` instead

### Additional context

_No response_

Start in datafusion/expr/src/expr_schema.rs at ExprSchemable::metadata and review the deprecation guidelines linked in the issue. Search for existing calls to metadata and inspect ExprSchemable::to_field documentation. Done means metadata is deprecated, to_field explains metadata access, and existing call sites use to_field.

Written by the indexing model from the issue text.