fix(datetime): selected today button renders correctly on ios (#28740)

Issue number: Internal

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## What is the current behavior?
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We removed the background color from today's calendar day button if
selected when implementing the calendar-day button shadow part feature:
https://github.com/ionic-team/ionic-framework/commit/79b005da704c2ce481e1e3bc4d24cdba06a36d04#diff-8e2912d52559aa44c9c6dc062c8d683e7b51c92c7b7ba420ad6f7587f1e1a61aL139

We did not catch this because we do not have test coverage for this use
case. We typically avoid rendering datetime on today's date because
"today" could be something totally different depending on the test
machine's locale/configuration.

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

- Adds the background rule that was removed
- Added test coverage. I mocked today's date to be a fixed date so it
never changes

## 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. -->

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