As already pointed in Greg's answer , mockito core is already included in spring-boot-starter-test so its already there in your project, no external dependency is needed.
Having said that - mockito's @Mock and Spring's @MockBean are a bit different in a way that @MockBean is basically a @Mock plus @Bean so it does what mockito annotation does plus it works with like a bean too i.e. it places instances in Spring's application context and all other spring bean related concepts come into picture too.