Modify Manage Files table header to reflect actual use of File's timestamp

[argiepiano]

This came up in #7173

The file table at admin/content/files has a column named "Uploaded". This label is misleading. The column displays the timestamp field of the File entities. That field does not store the uploaded date, but rather the last time the file entity was saved.

I suggest we change this to "Last saved", or "Updated" to match the column in the node and comment tables in that same UI.