Adopt to changes in Spring Framework 7.

See #2449

Author: mp911de

spring-data-rest-tests/spring-data-rest-tests-jpa/src/test/java/org/springframework/data/rest/webmvc/RepositorySearchControllerIntegrationTests.java

@@ -235,6 +235,6 @@ void returnsSimpleResponseEntityForQueryMethod() {
 				"findCreatedDateByLastName", PAGEABLE, Sort.unsorted(), new HttpHeaders(), assembler);
 
 		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);
-		assertThat(entity.getHeaders()).isEmpty();
+		assertThat(entity.getHeaders().isEmpty()).isTrue();
 	}
 }

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/support/ETagUnitTests.java

@@ -135,7 +135,7 @@ void doesNotAddHeaderForNoETag() {
 
 		HttpHeaders headers = ETag.NO_ETAG.addTo(new HttpHeaders());
 
-		assertThat(headers.containsKey("ETag")).isFalse();
+		assertThat(headers.getFirst("ETag")).isNull();
 	}
 
 	// tag::versioned-sample[]