[Modal] Fix can't override Backdrop Props using new Slots API

[ZeeshanTamboli]

• [x] I have followed (at least) the PR section of the contributing guide.

Fixes #35094

Note that the slotProps prop did not get updated like the componentsProps prop with the PropTypes generation script. I don't know much about that script so if you can point me out where to look at I can take a look.

[mui-bot]

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[mnajdova]

cc @michaldudak you may want to take a look at this.

[ZeeshanTamboli]

@michaldudak Can you take a look?

[ZeeshanTamboli]

The proptypes gerneration script treats slotProps the same way as componentsProps

Yes. But the slotProps proptype had not updated with the script was what I meant. Anyway, ignore this now since the proptypes are not needed to be updated.

[ZeeshanTamboli]

@michaldudak Ready for further review.