Supports parsing dates with UTC as zone-id

Such as: `2031-12-03T10:15:30.123456789Z[UTC]`

Author: jycr

server/src/main/java/org/elasticsearch/common/time/Iso8601Parser.java

@@ -450,7 +450,7 @@ private ZoneId parseZoneId(CharSequence str, int pos) {
 
         boolean positive;
         switch (first) {
-            case '+' -> positive = true;
+            case '+', 'Z' -> positive = true;
             case '-' -> positive = false;
             default -> {
                 // non-trivial zone offset, fallback on the built-in java zoneid parser
@@ -462,6 +462,9 @@ private ZoneId parseZoneId(CharSequence str, int pos) {
             }
         }
         pos++;  // read the + or -
+        if (str.charAt(pos) == '[' && str.charAt(len - 1) == ']') {
+            return parseRawZoneId(str, pos);
+        }
 
         Integer hours = parseInt(str, pos, pos += 2);
         if (hours == null || hours > 23) return null;

server/src/test/java/org/elasticsearch/common/time/DateFormattersTests.java

@@ -615,6 +615,7 @@ public void testParsingDateTimeWithZoneId() {
         formatter.format(formatter.parse("2031-12-03T10:15:30.123456789+01:00:00[Europe/Paris]"));
         formatter.format(formatter.parse("2031-12-03T10:15:30.123456789+01:00:00[PST]"));
         formatter.format(formatter.parse("2031-12-03T10:15:30.123456789+01:00:00[EST]"));
+        formatter.format(formatter.parse("2031-12-03T10:15:30.123456789Z[UTC]"));
     }
 
     public void testRoundupFormatterWithEpochDates() {