[core][Modal] Remove unnecessary manager prop handling

[ZeeshanTamboli]

While reviewing a PR I noticed the Modal component and useModal hook don't have a manager prop, and we don’t export useModal. This PR removes the manager prop handling and directly uses the default modal manager, in turn improving type safety when using the modal manager methods.

[mui-bot]

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[ZeeshanTamboli]

I had to cast it because the container prop is typed as Element (see here), but the modal manager methods expect HTMLElement, which is correct. Also, document.body (the default container) is an HTMLElement. Changing the container type in Portal would be a breaking change, though we could also allow both Element and HTMLElement.

[ZeeshanTamboli]

Can I get a review on this?

[aarongarciah]

Looks good. Not sure how we could get rid of the type casting without a breaking change.

[oliviertassinari]

This prop <Modal manager={}> was kind of a public API but it was never very clear, e.g. #14980 (comment). We will see if people have use cases to ask for this back.

[aarongarciah]

@oliviertassinari ouch. Sadly there wasn't a test to catch it.

[oliviertassinari]

It's Ok, we can wait to see if people create issues. As far as I remember, the only use case left for this manager prop is for people who load multiple versions of Material UI, and still need the modal manager singleton to correctly handle the top level modal. It was never really documented, more as workaround on issues. If we can drop that prop, great.

@atomiks As I understand it, in Base UI, the modal relies on Floating UI, https://master--base-ui.netlify.app/components/react-dialog/#nested-dialogs. And simply say: let's not fix it: https://github.com/mui/base-ui/blob/f51b2f4587df29a1f0d093745f4b8ed7439aba34/packages/mui-base/src/Dialog/Popup/DialogPopup.tsx#L33. So if we can get away with this in Material UI, great, we reduce the gap with Base UI.