feat(backdrop): ionic theme styles (#29873)

Issue number: internal

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## What is the current behavior?
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No Ionic theme styles for the `ion-backdrop` element

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

Adds Ionic theme styles for `ion-backdrop` and updates snapshots.

## 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. -->

The backdrop changes were only applied to `ion-modal` for now as all
other components where `ion-backdrop` is used do not yet have dedicated
Ionic theme styles. This is relevant as those components will need to
set `--backdrop-opacity: <value>` for their own implementations.