[upload] Implement theme attribute propagation

[javier-godoy]

Description

Implement theme attribute propagation for vaadin-file-upload items inside of vaadin-upload

Expected outcome

The theme attribute of vaadin-upload should be propagated into the vaadin-file-upload items.

Actual outcome

The theme attribute is not propagated.

Steps to reproduce

Try the following example;

Upload upload = new Upload();
upload.getElement().getThemeList().add("custom-theme");
upload.setMaxFiles(100);
upload.setAutoUpload(false);
add(upload);

[web-padawan]

In Vaadin 24 we moved vaadin-upload-file elements to light DOM in https://github.com/vaadin/web-components/pull/4870. So they can be styled using regular CSS now.

[web-padawan]

Closing per above comment as there is no need for theme attribute propagation in the V24 version of the component.