Assertions 사용 질문

package jpabook.jpashop;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.annotation.Rollback;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.transaction.annotation.Transactional;

@RunWith(SpringRunner.class)
@SpringBootTest
public class MemberRepositoryTest {
    @Autowired
    MemberRepository memberRepository;

    @Test
    @Transactional
    @Rollback(false)
    public void testMember() throws Exception {
        //given
        Member member = new Member();
        member.setUsername("memberA");

        //when
        Long savedId = memberRepository.save(member);

        //then
        Member findMember = memberRepository.find(savedId);

        Assertions.assertThat(findMember.getId()).isEqualTo(member.getId());
        Assertions.assertThat(findMember.getUsername()).isEqualTo(member.getUsername());
        Assertions.assertThat(findMember).isEqualTo(member);
    }
}

plugins {
    id 'java'
    id 'org.springframework.boot' version '3.2.1'
    id 'io.spring.dependency-management' version '1.1.4'
}

group = 'jpabook'
version = '0.0.1-SNAPSHOT'

java {
    sourceCompatibility = '21'
}

configurations {
    compileOnly {
       extendsFrom annotationProcessor
    }
}

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-data-jpa'
    implementation 'org.springframework.boot:spring-boot-starter-thymeleaf'
    implementation 'org.springframework.boot:spring-boot-starter-web'
    developmentOnly 'org.springframework.boot:spring-boot-devtools'
    testImplementation 'junit:junit:4.13'
    compileOnly 'org.projectlombok:lombok'
    runtimeOnly 'com.h2database:h2'
    annotationProcessor 'org.projectlombok:lombok'
    testImplementation 'org.springframework.boot:spring-boot-starter-test'

    //JUnit4 추가
    testImplementation("org.junit.vintage:junit-vintage-engine") {
       exclude group: "org.hamcrest", module: "hamcrest-core"
    }
}

tasks.named('test') {
    useJUnitPlatform()
}

이렇게 junit4 사용 하였는데 Assertions 이 구문만 쓰면 에러가 생기는데 왜 그런거죠?

import org.assertj.core.api.Assertions;