fix(form): support non-UTC timezone date time in VaadinDateTimePicker

[platosha]

Existing for VaadinDateTimePickerStrategy failed when running in non-UTC timezone environment. This change addresses the bug and makes the form library tests always use a non-UTC timezone.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 24.5 #2882 +/- ## ======================================= Coverage ? 92.31% ======================================= Files ? 81 Lines ? 2692 Branches ? 688 ======================================= Hits ? 2485 Misses ? 156 Partials ? 51 ``` | [Flag](https://app.codecov.io/gh/vaadin/hilla/pull/2882/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/vaadin/hilla/pull/2882/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | `92.31% <100.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin#carryforward-flags-in-the-pull-request-comment) to find out more.

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