fix(popover): content inside of popover scrolls correctly (#28861)

Issue number: resolves #28455

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## What is the current behavior?
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The main `ion-content` inside of a popover is not scrollable. Instead,
the `.popover-viewport` container is scrolled because certain styles no
longer applied once we moved popover to the Shadow DOM. This bug
impacted the linked issue because it meant that the infinite scroll
inside of the content never fires (since the content never scrolls).

## What is the new behavior?
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this PR. -->

- The `.popover-viewport` rule now targets slotted content with
the`.popover-viewport` class. This class is added to the root node that
is slotted. This enables us to target the user's content in the light
dom.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

Dev build: `7.6.6-dev.11705696534.177666f8`

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Co-authored-by: ionitron <hi@ionicframework.com>

core/src/components/popover/popover.scss

@@ -93,7 +93,7 @@
   z-index: $z-index-overlay-wrapper;
 }
 
-.popover-viewport {
+::slotted(.popover-viewport) {
   --ion-safe-area-top: 0px;
   --ion-safe-area-right: 0px;
   --ion-safe-area-bottom: 0px;