Use weekyear for consistency

Relates: elastic#60044 and elastic#59555 (review)

With the removal of camel case format names,
the format name for `WEEK_YEAR` has been set
to `week_year`, but this is inconsistent with the usage
in other formats related to weekyear e.g.

```
WEEKYEAR_WEEK("weekyear_week"),
WEEKYEAR_WEEK_DAY("weekyear_week_day"),
```

This commit

- replaces the snakecase format `week_year` with `weekyear`
- renames enum constants from `WEEK_YEAR` to `WEEKYEAR` for consistency
  with other constants related to weekyear.

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

@@ -1170,7 +1170,7 @@ public class DateFormatters {
     /*
      * Returns a formatter for a four digit weekyear. (YYYY)
      */
-    private static final DateFormatter WEEK_YEAR = new JavaDateFormatter("week_year",
+    private static final DateFormatter WEEKYEAR = new JavaDateFormatter("weekyear",
         new DateTimeFormatterBuilder().appendValue(WEEK_FIELDS_ROOT.weekBasedYear()).toFormatter(Locale.ROOT)
                                       .withResolverStyle(ResolverStyle.STRICT));
 
@@ -1720,9 +1720,9 @@ public class DateFormatters {
             return WEEK_DATE_TIME;
         } else if (FormatNames.WEEK_DATE_TIME_NO_MILLIS.matches(input)) {
             return WEEK_DATE_TIME_NO_MILLIS;
-        } else if (FormatNames.WEEK_YEAR.matches(input)) {
-            return WEEK_YEAR;
-        } else if (FormatNames.WEEK_YEAR_WEEK.matches(input)) {
+        } else if (FormatNames.WEEKYEAR.matches(input)) {
+            return WEEKYEAR;
+        } else if (FormatNames.WEEKYEAR_WEEK.matches(input)) {
             return WEEKYEAR_WEEK;
         } else if (FormatNames.WEEKYEAR_WEEK_DAY.matches(input)) {
             return WEEKYEAR_WEEK_DAY;

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

@@ -58,8 +58,8 @@ public enum FormatNames {
     WEEK_DATE("week_date"),
     WEEK_DATE_TIME("week_date_time"),
     WEEK_DATE_TIME_NO_MILLIS("week_date_time_no_millis"),
-    WEEK_YEAR("week_year"),
-    WEEK_YEAR_WEEK("weekyear_week"),
+    WEEKYEAR("weekyear"),
+    WEEKYEAR_WEEK("weekyear_week"),
     WEEKYEAR_WEEK_DAY("weekyear_week_day"),
     YEAR("year"),
     YEAR_MONTH("year_month"),

server/src/test/java/org/elasticsearch/common/joda/JavaJodaTimeDuellingTests.java

@@ -501,9 +501,9 @@ public void testDuellingFormatsValidParsing() {
         assertSameDate("2012-1-31", "year_month_day");
         assertSameDate("2012-12-1", "year_month_day");
 
-        assertSameDate("2018", "week_year");
-        assertSameDate("1", "week_year");
-        assertSameDate("2017", "week_year");
+        assertSameDate("2018", "weekyear");
+        assertSameDate("1", "weekyear");
+        assertSameDate("2017", "weekyear");
 
         assertSameDate("2018-W29", "weekyear_week");
         assertSameDate("2018-W1", "weekyear_week");
@@ -778,7 +778,7 @@ public void testSamePrinterOutput() {
         assertSamePrinterOutput("week_date", javaDate, jodaDate);
         assertSamePrinterOutput("week_date_time", javaDate, jodaDate);
         assertSamePrinterOutput("week_date_time_no_millis", javaDate, jodaDate);
-        assertSamePrinterOutput("week_year", javaDate, jodaDate);
+        assertSamePrinterOutput("weekyear", javaDate, jodaDate);
         assertSamePrinterOutput("weekyear_week", javaDate, jodaDate);
         assertSamePrinterOutput("weekyear_week_day", javaDate, jodaDate);
         assertSamePrinterOutput("year", javaDate, jodaDate);