This PR was merged into the 4.3-dev branch.

Discussion
----------

[Validator] Allow intl timezones

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | ref #28836
| License       | MIT
| Doc PR        | symfony/symfony-docs#11502

This PR considers the ICU timezones (#28831) as valid, as well as the PHP ones.

Both can be used in `DateTimeZone`, but expired timezones cannot be used with `IntlTimeZone`. The `intlCompatibility` option ensures compatibility between both.

Practically this allows for both `UTC` and `Etc/UTC`, where the latter should be preferred. I.e. currently `Etc/UTC` is considered an invalid timezone ... which it's not.

Commits
-------

7294b59 [Validator] Allow intl timezones