The [GMT timezone has been split from the UTC](http://site.icu-project.org/download/59) timezone [in CLDR](http://cldr.unicode.org/index/downloads/cldr-31) (which ICU is based on).
For example, the code blow:
* before ICU 59.1 would return "GMT" in all cases
* with ICU 59.1 it returns "UTC" for the first three ('z', 'zz', 'zzz')
and "Coordinated Universal Time" for the last two ('zzzz', 'zzzzz').
```php
foreach (['z', 'zz', 'zzz', 'zzzz', 'zzzzz'] as $pattern) {
$formatter = new \IntlDateFormatter('en', IntlDateFormatter::MEDIUM, IntlDateFormatter::SHORT, new \DateTimeZone('UTC'), IntlDateFormatter::GREGORIAN, $pattern);
var_dump($formatter->format(new \DateTime('@0')));
}
```
Similarly Form's `DateTimeToLocalizedStringTransformer` is also affected:
```php
$transformer = new DateTimeToLocalizedStringTransformer('UTC', 'UTC', null, \IntlDateFormatter::FULL);
var_dump($transformer->transform(new \DateTime('2010-02-03 04:05:06 UTC')));
// ICU 58.2: '03.02.2010, 04:05:06 GMT'
// ICU 59.1: '03.02.2010, 04:05:06 Koordinierte Weltzeit'
```
Refer to added and modified test cases for more changes. I split this PR in two commits for easier review. First commit updates ICU data (generated files), the second updates code and test cases to be compatible with updated data.
