Modernize java.time API usages

See gh-35861
Signed-off-by: Vincent Potucek <vpotucek@me.com>

spring-test/src/main/java/org/springframework/test/http/HttpHeadersAssert.java

@@ -18,6 +18,7 @@ package org.springframework.test.http;
 
 import java.time.Instant;
 import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
 import java.time.temporal.ChronoUnit;
 import java.util.Arrays;
@@ -42,7 +43,7 @@ import org.springframework.http.HttpHeaders;
  */
 public class HttpHeadersAssert extends AbstractObjectAssert<HttpHeadersAssert, HttpHeaders> {
 
-	private static final ZoneId GMT = ZoneId.of("GMT");
+	private static final ZoneId GMT = ZoneOffset.UTC;
 
 
 	private final AbstractCollectionAssert<?, Collection<? extends String>, String, ObjectAssert<String>> namesAssert;
@@ -173,7 +174,7 @@ public class HttpHeadersAssert extends AbstractObjectAssert<HttpHeadersAssert, H
 		containsHeader(name);
 		Assertions.assertThat(this.actual.getFirstZonedDateTime(name))
 				.as("check primary date value for HTTP header '%s'", name)
-				.isCloseTo(ZonedDateTime.ofInstant(value, GMT), Assertions.within(999, ChronoUnit.MILLIS));
+				.isCloseTo(value.atZone(GMT), Assertions.within(999, ChronoUnit.MILLIS));
 		return this.myself;
 	}
 

spring-test/src/test/java/org/springframework/test/web/servlet/client/RestTestClientTests.java

@@ -18,7 +18,7 @@ package org.springframework.test.web.servlet.client;
 
 import java.net.URI;
 import java.nio.charset.StandardCharsets;
-import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
 import java.util.Map;
 
@@ -263,7 +263,7 @@ class RestTestClientTests {
 		@Test
 		void testIfModifiedSince() {
 			RestTestClientTests.this.client.get().uri("/test")
-					.ifModifiedSince(ZonedDateTime.of(1970, 1, 1, 0, 0, 0, 0, ZoneId.of("GMT")))
+					.ifModifiedSince(ZonedDateTime.of(1970, 1, 1, 0, 0, 0, 0, ZoneOffset.UTC))
 					.exchange()
 					.expectStatus().isOk()
 					.expectBody().jsonPath("$.headers.If-Modified-Since").isEqualTo("Thu, 01 Jan 1970 00:00:00 GMT");

spring-test/src/test/java/org/springframework/test/web/servlet/result/HeaderResultMatchersTests.java

@@ -16,7 +16,7 @@
 
 package org.springframework.test.web.servlet.result;
 
-import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
 
 import org.junit.jupiter.api.Test;
@@ -45,7 +45,7 @@ public class HeaderResultMatchersTests {
 	@Test // SPR-17330
 	public void matchDateFormattedWithHttpHeaders() throws Exception {
 
-		long epochMilli = ZonedDateTime.of(2018, 10, 5, 0, 0, 0, 0, ZoneId.of("GMT")).toInstant().toEpochMilli();
+		long epochMilli = ZonedDateTime.of(2018, 10, 5, 0, 0, 0, 0, ZoneOffset.UTC).toInstant().toEpochMilli();
 		HttpHeaders headers = new HttpHeaders();
 		headers.setDate("myDate", epochMilli);
 		this.response.setHeader("d", headers.getFirst("myDate"));

spring-test/src/test/java/org/springframework/test/web/support/HeaderAssertionTests.java

@@ -17,7 +17,7 @@
 package org.springframework.test.web.support;
 
 import java.net.URI;
-import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
 import java.util.concurrent.TimeUnit;
 
@@ -272,7 +272,7 @@ class HeaderAssertionTests {
 	@Test
 	void expires() {
 		HttpHeaders headers = new HttpHeaders();
-		ZonedDateTime expires = ZonedDateTime.of(2018, 1, 1, 0, 0, 0, 0, ZoneId.of("UTC"));
+		ZonedDateTime expires = ZonedDateTime.of(2018, 1, 1, 0, 0, 0, 0, ZoneOffset.UTC);
 		headers.setExpires(expires);
 		TestHeaderAssertions assertions = new TestHeaderAssertions(headers);
 		assertions.expires(expires.toInstant().toEpochMilli());
@@ -285,7 +285,7 @@ class HeaderAssertionTests {
 	@Test
 	void lastModified() {
 		HttpHeaders headers = new HttpHeaders();
-		ZonedDateTime lastModified = ZonedDateTime.of(2018, 1, 1, 0, 0, 0, 0, ZoneId.of("UTC"));
+		ZonedDateTime lastModified = ZonedDateTime.of(2018, 1, 1, 0, 0, 0, 0, ZoneOffset.UTC);
 		headers.setLastModified(lastModified.toInstant().toEpochMilli());
 		TestHeaderAssertions assertions = new TestHeaderAssertions(headers);
 		assertions.lastModified(lastModified.toInstant().toEpochMilli());