Remove Timex from Plausible.Timezones

[ruslandoga]

Continues https://github.com/plausible/analytics/pull/4338

Changes

This PR continues the removal of Timex by updating Plausible.Timezones to use stdlib instead of Timex.

This includes:

• replacing Timex.Timezone.convert/2 with DateTime.shift_zone/2
• replacing Timex.to_datetime/2 with DateTime.from_naive/2 or DateTime.new/3 (for date -> datetime)
• replacing Timex.to_date/1 with DateTime.to_date/1

Unlike the previous PRs this one requires updates to the tests since Timex logic is wrong:

• Timex does not handle gaps in wall time: https://github.com/plausible/analytics/pull/3899#discussion_r1525956437
• Timex confuses + and - in Etc/GMT timezones: https://github.com/bitwalker/timex/issues/685

Tests

• [x] Automated tests have been updated

Changelog

• [x] No need to update changelog.

Documentation

• [x] This change does not need a documentation update

Dark mode

• [x] This PR does not change the UI

[github-actions[bot]]

Preview environment👷🏼‍♀️🏗️
PR-3899