Theme's post updates within update.php refer to themes as "module"

Problem/Motivation

Themes now support post update functions. See https://www.drupal.org/node/3259199

However when visiting /update.php to process the theme's update function, it refers to the theme as a module.

I expect the update function to refer to the theme as a theme.

Steps to reproduce

Proposed resolution

Change DbUpdateController to track updated by extension type, to not always assume that the extension is a module.

Remaining tasks

Commit

User interface changes

Before

After screenshot

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A