Describe the bug
The changing in casing of the onfocusin and onfocusout attributes to onFocusIn and onFocusOut in 4307, introduced in 10.19.7, breaks those types of event listeners.

To Reproduce

1. Go to https://stackblitz.com/edit/vitejs-vite-8tupqq?file=src%2Fmain.tsx
2. Click on the top input field, then click on the next input field
3. See that the top most component does not change from "No focus" to "Has focus". The focusin event listener is never triggered.

Inspecting the element using dev tools, the onFocusIn attrbute creates an event listener for the non standard FocusIn event instead of focusin.

Expected behavior
The same behavior of setting onFocusIn as the previous onfocusin, meaning event listeners for the focusin
since the latter has been removed from the declaration file.