MDL-75641 core_lang: Add string for new Europe/Kyiv timezone

With IANA TZDB (timezone database) release 2022b, Europe/Kiev
has been renamed to Europe/Kyiv. So we need to add this string
to our list to keep tests passing.

Note that the old Europe/Kiev continues working (is an alias) and,
also, there are places in core where we are using it. Just we cannot
remove/replace those uses in core until we ensure that everybody is
using PHP 8.0.23 (TZDB 2022b) or more recent versions. Hence,
not changing those uses now.

lang/en/timezones.php

@@ -363,6 +363,7 @@
 $string['europe/kaliningrad'] = 'Europe/Kaliningrad';
 $string['europe/kiev'] = 'Europe/Kiev';
 $string['europe/kirov'] = 'Europe/Kirov';
+$string['europe/kyiv'] = 'Europe/Kyiv';
 $string['europe/lisbon'] = 'Europe/Lisbon';
 $string['europe/ljubljana'] = 'Europe/Ljubljana';
 $string['europe/london'] = 'Europe/London';