fix(select): popover can be scrolled (#28965)

Issue number: resolves #28963

---------

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## What is the current behavior?
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In #28861 I fixed an
issue that caused the wrong content inside of a popover to be
scrollable. This CSS should have been applied, but it broke back when
popover was converted to the Shadow DOM. Fixing this issue revealed a
misconfiguration with the select-popover that caused the select-popover
to no longer be scrollable.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Content inside of `ion-select-popover` can now be scrolled

Note that I am considering this a bug fix instead of a regression. While
scrolling used to work in select-popover, it only worked by chance. The
`.popover-viewport` styles should have always applied to the
select-popover, thus requiring the use of `overflow-y: auto` in
select-popover.

## 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: `7.7.1-dev.11706893059.1bef4b38`

core/src/components/select-popover/select-popover.scss

@@ -8,3 +8,14 @@ ion-list-header,
 ion-label {
   @include margin(0);
 }
+
+
+/**
+ * The main content inside of a popover has overflow: hidden
+ * so that a descendant ion-content can be scrolled.
+ * However, select-popover does not use ion-content so we need
+ * to explicitly make this container scrollable.
+ */
+:host {
+  overflow-y: auto;
+}