refactor(content, modal): update prefix for internal safe area variable (#31114)

Issue number: N/A

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## What is the current behavior?
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We've decided to use `internal` as the prefix for internal CSS variables
and `ion` for public variables. We recently introduced a new internal
variable for the safe area that doesn't have the correct prefix.

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

- Updated `--ion-content-safe-area-padding-bottom` to
`--internal-content-safe-area-padding-bottom`

## 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. -->

N/A