ls: rename display_file_name to display_item_name

[cakebaker]

This PR renames the function display_file_name to display_item_name because the function is not limited to file names.

[sylvestre]

The doc gen is failing with:

error: unresolved link to `display_file_name`
    --> src/uu/ls/src/ls.rs:2547:22
     |
2547 | /// * `file_name` ([`display_file_name`])
     |                      ^^^^^^^^^^^^^^^^^ no item named `display_file_name` in scope
     |
     = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate
GNU test failed: tests/tail/truncate. tests/tail/truncate is passing on 'main'. Maybe you have to rebase?