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/SystemClock.java

@@ -31,10 +31,9 @@ public class SystemClock implements Clock {
 
   @Override
   public Instant now() {
-    // Truncate to milliseconds to match the precision of Joda DateTime and our database schema
-    // (which uses millisecond precision via DateTimeConverter). This prevents subtle comparison
-    // bugs where a high-precision Instant would be considered "after" a truncated database
-    // timestamp.
+    // Truncate to milliseconds to match the precision of our database schema.
+    // This prevents subtle comparison bugs where a high-precision Instant would be
+    // considered "after" a truncated database timestamp.
     return Instant.now().truncatedTo(MILLIS);
   }
 }