Prepare "Classes" for conversion to folders

[jkmassel]

Prerequisite for #23629 – we can't adopt folders until we drop CocoaPods, but all of the hard work can be done in advance.

This PR aligns the file system and the project.

To Test:

• Ensure this PR builds in CI
• Open the project in Xcode, right-click on the "Classes" directory, hold Option, then click Preflight Conversion to Folder. Note that it says “Classes” is ready to be converted to a folder.

[dangermattic]

	3 Warnings
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.
⚠️	This PR is larger than 500 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.
⚠️	This PR is assigned to the milestone 25.5. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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