What does this PR change? What problem does it solve?

The ls command was the only command that used the struct_type field to determine
the message type in the JSON output format. Now, the JSON output of the
ls command also includes the message_type. The struct_type field is
still included, but it deprecated.

Was the change previously discussed in an issue or on the forum?

No.

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.