Fix #68471: IntlDateFormatter fails for "GMT+00:00" timezone

GMT+00:00 is recognized by ICU, and is normalized to GMT. There are no
issues when GMT+00:00 is passed to `IntlTimeZone::cre

Fix #68471: IntlDateFormatter fails for "GMT+00:00" timezone

GMT+00:00 is recognized by ICU, and is normalized to GMT. There are no
issues when GMT+00:00 is passed to `IntlTimeZone::createTimeZone()`,
but passing it to IntlDateFormatter::__construct() causes a failure,
since there is an additional check regarding the validity. While
checking the validity of the result of `TimeZone::createTimeZone()`[1]
is a good idea, comparing the IDs is overly restrictive. Instead we
just check that the timezone is supported by ICU.

[1] <https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/classicu_1_1TimeZone.html#a35da0507b62754ffe5d8d59c19775cdb>

Closes GH-7190.

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