refactor(core): use Capacitor safe-area CSS variables on older WebViews (#30866)

Issue number: internal

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## What is the current behavior?
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The safe area variables are only reliant on `env` variables that are
provided by devices.

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

Capacitor 8 has released [safe area variable
fallbacks](https://capacitorjs.com/docs/apis/system-bars#android-note)
to provide consistent behaviors with older Android devices:

> Due to a [bug](https://issues.chromium.org/issues/40699457) in some
older versions of Android WebView (< 140), correct safe area values are
not available via the safe-area-inset-x CSS env variables. This plugin
will inject the correct inset values into a new CSS variable(s) named
--safe-area-inset-x that you can use as a fallback in your frontend
styles.

- Updated safe area variables to use the fallbacks provided by
Capacitor.

## 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. -->

Changes to the [core
file](https://github.com/ionic-team/ionic-framework/blob/2ee52d77c86ef29756370f1ed732998589ca2fd8/core/src/css/core.scss#L253-L260)
will be done on `main`.

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Co-authored-by: Brandy Smith <6577830+brandyscarney@users.noreply.github.com>
Co-authored-by: ionitron <hi@ionicframework.com>