Complete Joda-Time to java.time migration (#3039)

This completes the exhaustive refactoring of foundational temporal types from Joda-Time to the native java.time API across the entire codebase.

- Replaced org.joda.time.DateTime, Instant, LocalDate, and Duration with java.time equivalents.
- Audited and updated Clock implementations (FakeClock, SystemClock). Added nowMillis(), nowDate(), and nowDateTime() to eliminate repetitive conversions and maintain parallel naming.
- Replaced ZonedDateTime with OffsetDateTime globally per go/avoid-zdt. OffsetDateTime is a better fit as we use a hardcoded ZoneOffset.UTC throughout the system, making geographical time zone rules (like daylight saving time) irrelevant and preventing serialization ambiguities. Added a presubmit check.
- Completely removed all transitional bridge methods from DateTimeUtils and deleted obsolete converters (e.g., DateTimeConverter).
- Updated testing infrastructure, Apache Beam pipelines, custom JCommander parameters, and networking modules to solely rely on java.time primitives.
- Retained the lone necessary org.joda.time.Instant usage in SafeBrowsingTransforms required by the Apache Beam API.
- Cleared Gradle lockfiles and removed the joda-time dependency entirely from the build configuration.

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

@@ -16,15 +16,16 @@ package google.registry.util;
 
 import java.io.Serializable;
 import java.time.Instant;
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
 import java.time.ZoneOffset;
-import java.time.ZonedDateTime;
 import javax.annotation.concurrent.ThreadSafe;
 
 /**
  * A clock that tells the current time in milliseconds or nanoseconds.
  *
  * <p>Clocks are technically serializable because they are either a stateless wrapper around the
- * system clock, or for testing, are just a wrapper around a DateTime. This means that if you
+ * system clock, or for testing, are just a wrapper around an Instant. This means that if you
  * serialize a clock and deserialize it elsewhere, you won't necessarily get the same time or time
  * zone -- what you will get is a functioning clock.
  */
@@ -34,8 +35,18 @@ public interface Clock extends Serializable {
   /** Returns current Instant (which is always in UTC). */
   Instant now();
 
-  /** Returns the current time as a {@link ZonedDateTime} in UTC. */
-  default ZonedDateTime nowDate() {
-    return ZonedDateTime.ofInstant(now(), ZoneOffset.UTC);
+  /** Returns the current time as an {@link OffsetDateTime} in UTC. */
+  default OffsetDateTime nowDateTime() {
+    return OffsetDateTime.ofInstant(now(), ZoneOffset.UTC);
+  }
+
+  /** Returns the current time as a {@link LocalDate} in UTC. */
+  default LocalDate nowDate() {
+    return LocalDate.ofInstant(now(), ZoneOffset.UTC);
+  }
+
+  /** Returns the current time in milliseconds since the epoch. */
+  default long nowMillis() {
+    return now().toEpochMilli();
   }
 }