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.

load-testing/src/main/java/google/registry/client/EppClientHandler.java

@@ -23,7 +23,6 @@ import static google.registry.client.EppClient.LOGGING_REQUEST_COMPLETE;
 import static google.registry.client.EppClient.REQUEST_SENT;
 import static google.registry.client.EppClient.RESPONSE_RECEIVED;
 import static java.nio.file.StandardOpenOption.APPEND;
-import static java.time.ZoneOffset.UTC;
 
 import com.google.common.flogger.FluentLogger;
 import io.netty.buffer.ByteBuf;
@@ -38,7 +37,7 @@ import io.netty.util.concurrent.Promise;
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
-import java.time.ZonedDateTime;
+import java.time.Instant;
 
 /** Handler that sends EPP requests and receives EPP responses. */
 @SuppressWarnings("FutureReturnValueIgnored")
@@ -51,9 +50,9 @@ public class EppClientHandler extends ChannelDuplexHandler {
 
     private final Path loggingLocation;
     private final byte[] contents;
-    private final ZonedDateTime time;
+    private final Instant time;
 
-    FileWriter(Path loggingLocation, byte[] contents, ZonedDateTime time) {
+    FileWriter(Path loggingLocation, byte[] contents, Instant time) {
       this.loggingLocation = loggingLocation;
       this.contents = contents;
       this.time = time;
@@ -75,7 +74,7 @@ public class EppClientHandler extends ChannelDuplexHandler {
 
   @Override
   public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
-    ZonedDateTime now = ZonedDateTime.now(UTC);
+    Instant now = Instant.now();
     ctx.channel().attr(REQUEST_SENT).get().add(now);
     ctx.channel().attr(LOGGING_REQUEST_COMPLETE).set(ctx.executor().newPromise());
     super.channelRegistered(ctx);
@@ -93,7 +92,7 @@ public class EppClientHandler extends ChannelDuplexHandler {
 
   @Override
   public void channelRead(ChannelHandlerContext ctx, Object msg) {
-    ZonedDateTime now = ZonedDateTime.now(UTC);
+    Instant now = Instant.now();
     Channel ch = ctx.channel();
     ctx.channel().attr(RESPONSE_RECEIVED).get().add(now);
     if (msg instanceof ByteBuf buffer) {
@@ -118,7 +117,7 @@ public class EppClientHandler extends ChannelDuplexHandler {
 
   @Override
   public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) {
-    ZonedDateTime now = ZonedDateTime.now(UTC);
+    Instant now = Instant.now();
     Channel ch = ctx.channel();
     ctx.channel().attr(REQUEST_SENT).get().add(now);
     if (msg instanceof byte[] outputBytes) {