xcm: Fixes for `UnpaidLocalExporter` (#7126)

## Description

This PR deprecates `UnpaidLocalExporter` in favor of the new
`LocalExporter`. First, the name is misleading, as it can be used in
both paid and unpaid scenarios. Second, it contains a hard-coded channel
0, whereas `LocalExporter` uses the same algorithm as `xcm-exporter`.

## Future Improvements  

Remove the `channel` argument and slightly modify the
`ExportXcm::validate` signature as part of [this
issue](https://github.com/orgs/paritytech/projects/145/views/8?pane=issue&itemId=84899273).

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Co-authored-by: command-bot <>