Refactor: delete unused media button

[amoniaka-knabino]

Purpose / Description

Change OK button to Delete button on "Delete unused media?" dialog window.

Fixes

• Part of Improve some dialogs (mostly string changes) #20508

Approach

More easy to understand button meaning

How Has This Been Tested?

Commit tests + UI check.

Learning (optional, can help others)

Describe the research stage

Links to blog posts, patterns, libraries or addons used to solve this problem

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[amoniaka-knabino]

Before

After

[criticalAY]

LGTM!

Here is something to learn @amoniaka-knabino : Fixes # would close the issue where as we are just trying to resolve a part of it (updated the description) and it would be good if we can learn rebase