Remove the MediaThumbnailService

[kean]

The PR #22042 (23.8) introduced the .medium size of the thumbnails for use by the Site Media screen, but this change is incomplete without removing the remaining usage of MediaThumbnailService in MediaImportService to pre-generate thumbnails for the use by the editor.

This PR removes the last use of the deprecated MediaThumbnailService and replaces with a new service. The thumbnails are slightly smaller than created by MediaThumbnailService to reduce the disk space usage, in the same vein as #21981.

To test:

• Verify that the editor displays an image placeholder during the upload

Regression Notes

1. Potential unintended areas of impact: Editor
2. What I did to test those areas of impact (or what existing automated tests I relied on): manual
3. What automated tests I added (or what prevented me from doing so): n/a

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

UI Changes testing checklist:

• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22106-2693a99
	Version	23.8
	Bundle ID	org.wordpress.alpha
	Commit	2693a99
	App Center Build	WPiOS - One-Offs #7984

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22106-2693a99
	Version	23.8
	Bundle ID	com.jetpack.alpha
	Commit	2693a99
	App Center Build	jetpack-installable-builds #7008

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

	1 Warning
⚠️	View files have been modified, but no screenshot is included in the pull request. Consider adding some for clarity.

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