refactor: match artwork filter feature flag name to project name

[iskounen]

The type of this PR is: Refactor

This PR resolves FX-2719

Description

I originally named the feature flag for the ongoing artwork filter work ARUseNewArtworkFilters in #4515. Upon reflection I would like to rename this to be ARUseImprovedArtworkFilters because:

1. That is the name of the JIRA epic and project brief for this work, and
2. The new artwork filters won't be new forever

PR Checklist (tick all before merging)

• I have included screenshots or videos to illustrate my changes, or I have not changed anything that impacts the UI.
• I have added tests for my changes, or my changes don't require testing, or I have included a link to a separate Jira ticket covering the tests.
• I have documented any follow-up work that this PR will require, or it does not require any.
• I have added an app state migration, or my changes do not require one. (What are migrations?)
• I have added a CHANGELOG.yml entry or my changes do not require one.

[iskounen]

This is a draft until #4507 is merged and I get a chance to include changes to that code in this PR.