fix(datetime): multiple month selected and flakiness display (#31053)

Issue number: resolves internal

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## What is the current behavior?
Currently, the Datepicker keeps all the months visited as selected
instead of only the one that is really selected
On iOS devices, the month picker sometimes doesn't appear since the
`datepicker-ready` class is not added

## What is the new behavior?
Only the selected month appears selected in the month picker
On iOS devices, an additional validation is done to add the
`datepicker-ready` class

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

The
[Basic](https://ionic-framework-7el9b9l3f-ionic1.vercel.app/src/components/datetime/test/basic/?ionic:theme=ionic)
test should be used to test the multiple-month selection case
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screenshots of how the component looks before and after the change. -->