feat(datetime): update weekdays abbreviation for ionic theme (#30172)

Issue number: resolves internal

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## What is the current behavior?
- Previously for Ionic theme, the weekdays in `ion-datetime` were
displayed in the "narrow" version, such as "M" or "T".

## What is the new behavior?
- Now, the weekdays are displayed in the "short" version: "Mon", "Tue",
etc.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

Before:

![image](https://github.com/user-attachments/assets/f2939274-369c-45c8-86e6-36dabc0cd510)

After:

![image](https://github.com/user-attachments/assets/8dca06e1-bc99-4d78-af79-bef2e5ea3fed)

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