Fix time zone issue in Rounding serialization (#50845)

When deserializing time zones in the Rounding classes we used to include a tiny
normalization step via `DateUtils.of(in.readString())` that was lost in #50609.
Its at least necessary for some tests, e.g. the cause of #50827 is that when
sending the default time zone ZoneOffset.UTC on a stream pre 7.0 we convert it
to a "UTC" string id via `DateUtils.zoneIdToDateTimeZone`. This gets then read
back as a UTC ZoneRegion, which should behave the same but fails the equality
tests in our serialization tests. Reverting to the previous behaviour with an
additional normalization step on 7.x.

Co-authored-by: Nik Everett <[email protected]>

Closes #50827

Author: Christoph Büscher

server/src/main/java/org/elasticsearch/common/Rounding.java

@@ -252,7 +252,8 @@ static class TimeUnitRounding extends Rounding {
         }
 
         TimeUnitRounding(StreamInput in) throws IOException {
-            this(DateTimeUnit.resolve(in.readByte()), in.readZoneId());
+            this(DateTimeUnit.resolve(in.readByte()),
+                    in.getVersion().onOrAfter(Version.V_7_0_0) ? in.readZoneId() : DateUtils.of(in.readString()));
         }
 
         @Override
@@ -467,7 +468,8 @@ static class TimeIntervalRounding extends Rounding {
         }
 
         TimeIntervalRounding(StreamInput in) throws IOException {
-            this(in.readVLong(), in.readZoneId());
+            this(in.readVLong(),
+                    in.getVersion().onOrAfter(Version.V_7_0_0) ? in.readZoneId() : DateUtils.of(in.readString()));
         }
 
         @Override

server/src/test/java/org/elasticsearch/search/aggregations/InternalAggregationsTests.java

@@ -120,7 +120,6 @@ public static InternalAggregations createTestInstance() throws Exception {
         return new InternalAggregations(aggsList, topLevelPipelineAggs);
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/50827")
     public void testSerialization() throws Exception {
         InternalAggregations aggregations = createTestInstance();
         writeToAndReadFrom(aggregations, 0);