fix(material/card): use more specific selector for divider override

[crisbeto]

Fixes that the card targets all dividers which may include ones placed inside child components.

Fixes #23524.

[crisbeto]

Caretaker note: this has a chance of screenshot failures if people didn't compose their card correctly.

[tmburnell]

I am the one that created the original issue, I want to point out one concern i had.
The fix above might break this case

because the divider is not a direct child of the mat-card, it is actually a child of the mat-list.
So i dont know if there needs to be an or so we dont break some people existing layouts.

[crisbeto]

I think that in that case the list should be the one responsible for the divider position.

[jelbourn]

LGTM

[Sergiobop]

Please merge this! Nice fix 👍

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