Enhancement: Make `ls` use `message_type` field in JSON output

The `ls` command was the only restic command that used the `struct_type` field
in its JSON output format to specify the message type.

The JSON output of the `ls` command now also includes the `message_type` field,
which is consistent with other commands. The `struct_type` field is still
included, but now deprecated.

https://github.com/restic/restic/pull/4664
