Refactor foundational temporal types to java.time (#3036)

* Migrates core classes (Clock, Sleeper, TransactionManager) and extensive domain models from Joda-Time to java.time.
* Restores original public API method names while substituting parameters/return values with `java.time.Instant`.
* Updates JAXB XJC `bindings.xjb` to natively generate `java.time.Instant` and `java.time.LocalDate`, eliminating `toDateTime` wrapper methods.
* Fixes XML serializers (`DateAdapter`) to robustly convert OffsetDateTime timezone strings to UTC.
* Cleans up redundant imports and Checkstyle failures across the codebase.

Remaining Joda-Time surface area to migrate in future tasks:
* Command-line parameters (e.g. `DateTimeParameter`, `DateParameter`, `IntervalParameter`) in `google.registry.tools.params`.
* EPP/RDAP flow testing infrastructure (`EppTestCase`, `RdapActionBaseTestCase`, `FlowTestCase`).
* Beam pipelines and Load Testing modules (`Spec11PipelineTest`, `RdePipelineTest`, `RegistryJpaReadTest`, `EppClient`).
* Utility bridges and converters (`DateTimeUtils.toDateTime/toInstant`, `DateTimeConverter`, `UtcDateTimeAdapter`).
* Remaining UI Console tests and Actions.

common/src/main/java/google/registry/util/Clock.java

@@ -19,7 +19,6 @@ import java.time.Instant;
 import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
 import javax.annotation.concurrent.ThreadSafe;
-import org.joda.time.DateTime;
 
 /**
  * A clock that tells the current time in milliseconds or nanoseconds.
@@ -32,10 +31,6 @@ import org.joda.time.DateTime;
 @ThreadSafe
 public interface Clock extends Serializable {
 
-  /** Returns current time in UTC timezone. */
-  @Deprecated
-  DateTime nowUtc();
-
   /** Returns current Instant (which is always in UTC). */
   Instant now();