Kalmar Lorand

[FocusTrap] update getTabbable function return type

3

Part of: [#42231](https://github.com/mui/material-ui/issues/42231) `FocusTrap` component: Change the return type of the `getTabbable` function from `ReadonlyArray` to `ReadonlyArray`.

[material-ui][FocusTrap] getTabbable function has a return type of string[] instead of HTMLElement[]

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### Steps to reproduce [Link to live example: (required)](https://codesandbox.io/p/sandbox/mui-focustrap-gettabbable-issue-9xc9jc) Steps: 1. Create a Focustrap component 2. Pass in the getTabbable function that returns an array of nodes [Returns an array...