"Modified" date column in Typesetting Plugin no longer populating

[pgoussy]

Describe the bug
In the Typesetting Plugin, the Modified column no longer displays the date the typeset file was last modified, as it used to. This occurs in both the top-level Typesetting Plugin ("Current Typeset Files" box) and in each individual galley's Edit Typeset File page. The latter feels more problematic in practical use, since there's no longer an indication of whether or not I've already edited a typeset file that I clicked in to replace.

Janeway version
Currently 1.4.1; I don't recall when it started happening, but it's been a couple of months at least (I just kept forgetting to report it).

To Reproduce
Steps to reproduce the behavior:

1. Go to the archive page of any published article. (e.g. https://journals.publishing.umich.edu/ergo/manage/archive/article/1113/)
2. Click the "Typesetting Plugin" button in the top menu bar. (e.g. https://journals.publishing.umich.edu/ergo/plugins/typesetting/article/1113/)
3. Scroll down to the Current Typeset Files.

Expected behavior
"Modified" column should display the date (and time?) that the typeset file was last modified.

Screenshots

[mauromsl]

This a side-effect of the changes made in #2804 (removed the old date_modified column from core_file)

We should add a property to to the model with a deprecation notice, preserving the old interface.

I've also raised an issue to update typesetting plugin: openlibhums/typesetting#158